Fix build error when CONFIG_NVME_AUTH=m (module):

  drivers/nvme/common/auth.c:158:13: error: redefinition of
  'nvme_auth_extract_key'
  include/linux/nvme-auth.h:25:27: note: previous definition is here

The issue occurs because when NVME_AUTH is built as a module (=m), the
preprocessor symbol CONFIG_NVME_AUTH is not defined during compilation,
but the object file auth.o is still compiled as part of nvme-auth.ko.

With #ifdef CONFIG_NVME_AUTH, the header provides a static inline stub
when the config is =m, while auth.c provides the real implementation,
causing a redefinition conflict.

Use IS_REACHABLE(CONFIG_NVME_AUTH) instead of #ifdef CONFIG_NVME_AUTH to
properly handle both built-in (=y) and modular (=m) builds. IS_REACHABLE
evaluates to true when the code is reachable, either because it's built-in
or because we're building the same module.

Fixes: e30cd923b3ed ("nvme-auth: switch to use 'struct key'")
Signed-off-by: Chaitanya Kulkarni <ckulkarnilinux@gmail.com>
---
Hi,

Please review this very carefully :-

Broken scenario with #ifdef CONFIG_NVME_AUTH:

make menuconfig -->  NVMe over Fabrics In-Band Authentication --> * 

nvme (nvme-6.19) # grep NVME_AUTH .config
CONFIG_NVME_AUTH_STATE=y
CONFIG_NVME_AUTH=m
nvme (nvme-6.19) #

1: Makefile compiles auth.c
  obj-$(CONFIG_NVME_AUTH) += nvme-auth.o
  nvme-auth-y += auth.o
  auth.c gets compiled as part of nvme-auth.ko module
  make[1]: Entering directory '/mnt/data/nvme/drivers/nvme'
  CC [M]  common/auth.o
  CC [M]  host/core.o
  CC [M]  host/sysfs.o
  CC [M]  target/configfs.o
  CC [M]  host/auth.o
  CC [M]  host/fabrics.o
  CC [M]  target/fabrics-cmd-auth.o
  CC [M]  target/auth.o
common/auth.c:158:13: error: redefinition of ‘nvme_auth_extract_key’
  158 | struct key *nvme_auth_extract_key(struct key *keyring, const u8 *secret,
      |             ^~~~~~~~~~~~~~~~~~~~~

2: Preprocessor evaluates header guards in nvme-auth.h

#ifdef CONFIG_NVME_AUTH <---- CONFIG_NVME_AUTH is NOT defined when =m
                                 Only CONFIG_NVME_AUTH_MODULE is defined!
  struct key *nvme_auth_extract_key(...); <---  Declaration (NOT reached)
#else
static inline struct key *nvme_auth_extract_key(...) { <---- This is used  Static inline stub
      return ERR_PTR(-ENOKEY);                           
  }
  #endif

3: Compiler sees BOTH definitions when NVME_AUTH is enabled
   using make menuconfig  NVMe over Fabrics In-Band Authentication = * 
  - auth.c:158 has the real implementation: struct key *nvme_auth_extract_key(...)
  - Header provides static inline stub: static inline struct key *nvme_auth_extract_key(...)

---
 include/linux/nvme-auth.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/nvme-auth.h b/include/linux/nvme-auth.h
index afc84a4cd94a..49a8afc16028 100644
--- a/include/linux/nvme-auth.h
+++ b/include/linux/nvme-auth.h
@@ -18,7 +18,7 @@ const char *nvme_auth_digest_name(u8 hmac_id);
 size_t nvme_auth_hmac_hash_len(u8 hmac_id);
 u8 nvme_auth_hmac_id(const char *hmac_name);
 
-#ifdef CONFIG_NVME_AUTH
+#if IS_REACHABLE(CONFIG_NVME_AUTH)
 struct key *nvme_auth_extract_key(struct key *keyring, const u8 *secret,
 				  size_t secret_len, bool *generated);
 #else
-- 
2.40.0