[PATCH][next] jbd2: Avoid dozens of -Wflex-array-member-not-at-end warnings

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

-Wflex-array-member-not-at-end was introduced in GCC-14, and we
are getting ready to enable it, globally.

Use the `DEFINE_RAW_FLEX()` helper for an on-stack definition of
a flexible structure (`struct shash_desc`) where the size of the
flexible-array member (`__ctx`) is known at compile-time, and
refactor the rest of the code, accordingly.

So, with this, fix 77 of the following warnings:

include/linux/jbd2.h:1800:35: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
 include/linux/jbd2.h | 13 +++++--------
 1 file changed, 5 insertions(+), 8 deletions(-)

diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
index 8aef9bb6ad57..ce4560e62d3b 100644
--- a/include/linux/jbd2.h
+++ b/include/linux/jbd2.h
@@ -1796,22 +1796,19 @@ static inline unsigned long jbd2_log_space_left(journal_t *journal)
 static inline u32 jbd2_chksum(journal_t *journal, u32 crc,
 			      const void *address, unsigned int length)
 {
-	struct {
-		struct shash_desc shash;
-		char ctx[JBD_MAX_CHECKSUM_SIZE];
-	} desc;
+	DEFINE_RAW_FLEX(struct shash_desc, desc, __ctx, 1);
 	int err;
 
 	BUG_ON(crypto_shash_descsize(journal->j_chksum_driver) >
 		JBD_MAX_CHECKSUM_SIZE);
 
-	desc.shash.tfm = journal->j_chksum_driver;
-	*(u32 *)desc.ctx = crc;
+	desc->tfm = journal->j_chksum_driver;
+	*(u32 *)desc->__ctx = crc;
 
-	err = crypto_shash_update(&desc.shash, address, length);
+	err = crypto_shash_update(desc, address, length);
 	BUG_ON(err);
 
-	return *(u32 *)desc.ctx;
+	return *(u32 *)desc->__ctx;
 }
 
 /* Return most recent uncommitted transaction */
-- 
2.34.1