xfs: log proper length of btree block in scrub/repair

xfs_trans_log_buf() takes a final argument of the last byte to
log in the buffer; b_length is in basic blocks, so this isn't
the correct last byte.  Fix it.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>