xfs: Add __packed to xfs_dir2_sf_entry_t definition

xfs_check_ondisk_structs() verifies that the sizes of the data types
used by xfs are correct via the XFS_CHECK_STRUCT_SIZE() macro.

Since the structures padding can vary depending on the ABI (e.g. on
ARM OABI structures are padded to multiple of 32 bits), it may happen
that xfs_dir2_sf_entry_t size check breaks the compilation with the
assertion below:

In file included from linux/include/linux/string.h:6,
                 from linux/include/linux/uuid.h:12,
                 from linux/fs/xfs/xfs_linux.h:10,
                 from linux/fs/xfs/xfs.h:22,
                 from linux/fs/xfs/xfs_super.c:7:
In function ‘xfs_check_ondisk_structs’,
    inlined from ‘init_xfs_fs’ at linux/fs/xfs/xfs_super.c:2025:2:
linux/include/linux/compiler.h:350:38:
    error: call to ‘__compiletime_assert_107’ declared with attribute
    error: XFS: sizeof(xfs_dir2_sf_entry_t) is wrong, expected 3
    _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)

Restore the correct behavior adding __packed to the structure definition.

Cc: Darrick J. Wong <darrick.wong@oracle.com>
Suggested-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/libxfs/xfs_da_format.h

@@ -217,7 +217,7 @@ typedef struct xfs_dir2_sf_entry {
 	 * A 64-bit or 32-bit inode number follows here, at a variable offset
 	 * after the name.
 	 */
-} xfs_dir2_sf_entry_t;
+} __packed xfs_dir2_sf_entry_t;
 
 static inline int xfs_dir2_sf_hdr_size(int i8count)
 {