xfs: make struct xfs_buf_log_format have a consistent size

Increase XFS_BLF_DATAMAP_SIZE by 1 to fill in the implied padding at the
end of struct xfs_buf_log_format.  This makes the size consistent so
that we can check it in xfs_ondisk.h, and will be needed once we start
logging attribute values.

On amd64 we get the following pahole:

struct xfs_buf_log_format {
        short unsigned int         blf_type;       /*     0     2 */
        short unsigned int         blf_size;       /*     2     2 */
        short unsigned int         blf_flags;      /*     4     2 */
        short unsigned int         blf_len;        /*     6     2 */
        long long int              blf_blkno;      /*     8     8 */
        unsigned int               blf_map_size;   /*    16     4 */
        unsigned int               blf_data_map[16]; /*    20    64 */
        /* --- cacheline 1 boundary (64 bytes) was 20 bytes ago --- */

        /* size: 88, cachelines: 2, members: 7 */
        /* padding: 4 */
        /* last cacheline: 24 bytes */
};

But on i386 we get the following:

struct xfs_buf_log_format {
        short unsigned int         blf_type;       /*     0     2 */
        short unsigned int         blf_size;       /*     2     2 */
        short unsigned int         blf_flags;      /*     4     2 */
        short unsigned int         blf_len;        /*     6     2 */
        long long int              blf_blkno;      /*     8     8 */
        unsigned int               blf_map_size;   /*    16     4 */
        unsigned int               blf_data_map[16]; /*    20    64 */
        /* --- cacheline 1 boundary (64 bytes) was 20 bytes ago --- */

        /* size: 84, cachelines: 2, members: 7 */
        /* last cacheline: 20 bytes */
};

Notice how the amd64 compiler inserts 4 bytes of padding to the end of
the structure to ensure 8-byte alignment.  Prior to "xfs: fix memory
corruption during remote attr value buffer invalidation" we would try to
write to blf_data_map[17], which is harmless on amd64 but really bad on
i386.

This shouldn't cause any changes in the ondisk logging formats because
the log code writes out the log vectors with the appropriate size for
the log item's map_size, and log recovery treats the data_map array as a
VLA.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>