[PATCH] idmap: override btrfs_ioctl_vol_args_v2 if system header doesn't have subvolid

["Darrick J. Wong" <djwong@kernel.org>]

From: Darrick J. Wong <djwong@kernel.org>

The struct btrfs_ioctl_vol_args_v2 in /usr/include/linux/btrfs.h on my
system predates the inclusion of the "subvolid" field.  This causes the
following build failure:

idmapped-mounts.c: In function 'btrfs_delete_subvolume_id':
idmapped-mounts.c:9730:6: error: 'struct btrfs_ioctl_vol_args_v2' has no member named 'subvolid'
 9730 |  args.subvolid = subvolid;
      |      ^

Since this source file contains its own more uptodate copy of that
structure, add some more autoconf/cpp magic so that we can override the
struct definition if the system header doesn't have the desired field.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
---
 configure.ac                          |    2 +-
 src/idmapped-mounts/idmapped-mounts.c |    6 +++++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index 4117a302..36960910 100644
--- a/configure.ac
+++ b/configure.ac
@@ -79,7 +79,7 @@ AC_CHECK_TYPES([struct btrfs_ioctl_ino_lookup_args], [], [], [[#include <linux/b
 AC_CHECK_TYPES([struct btrfs_ioctl_ino_lookup_user_args], [], [], [[#include <linux/btrfs.h>]])
 AC_CHECK_TYPES([struct btrfs_ioctl_get_subvol_rootref_args], [], [], [[#include <linux/btrfs.h>]])
 AC_CHECK_HEADERS([linux/btrfs.h linux/btrfs_tree.h])
-
+AC_CHECK_MEMBERS([struct btrfs_ioctl_vol_args_v2.subvolid], [], [], [[#include <linux/btrfs.h>]])
 
 AC_CONFIG_HEADER(include/config.h)
 AC_CONFIG_FILES([include/builddefs])
diff --git a/src/idmapped-mounts/idmapped-mounts.c b/src/idmapped-mounts/idmapped-mounts.c
index 78b7cc09..40eb2cc6 100644
--- a/src/idmapped-mounts/idmapped-mounts.c
+++ b/src/idmapped-mounts/idmapped-mounts.c
@@ -23,7 +23,11 @@
 #include <unistd.h>
 
 #ifdef HAVE_LINUX_BTRFS_H
+# ifndef HAVE_STRUCT_BTRFS_IOCTL_VOL_ARGS_V2_SUBVOLID
+#  define btrfs_ioctl_vol_args_v2 override_btrfs_ioctl_vol_args_v2
+# endif
 #include <linux/btrfs.h>
+# undef btrfs_ioctl_vol_args_v2
 #endif
 
 #ifdef HAVE_LINUX_BTRFS_TREE_H
@@ -9570,7 +9574,7 @@ struct btrfs_qgroup_inherit {
 };
 #endif
 
-#ifndef HAVE_STRUCT_BTRFS_IOCTL_VOL_ARGS_V2
+#if !defined(HAVE_STRUCT_BTRFS_IOCTL_VOL_ARGS_V2) || !defined(HAVE_STRUCT_BTRFS_IOCTL_VOL_ARGS_V2_SUBVOLID)
 
 #ifndef BTRFS_SUBVOL_NAME_MAX
 #define BTRFS_SUBVOL_NAME_MAX 4039