xfs_repair: take the ag_lock before recording rmap for a bmbt record

xfs_repair: take the ag_lock before recording rmap for a bmbt record

When the (threaded) inode scanner iterates the blocks of a bmbt tree and
wants to record the bmbt blocks in the in-core rmap database, we have to
take the ag_lock for the AG that the bmbt block is in, or else we can
accidentally corrupt the rmap slab by calling slab_add from two threads.

Reported-by: matorola@gmail.com
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>