xfs: specify AG in bulk req

Add a new xfs_bulk_ireq flag to constrain the iteration to a single AG.
If the passed-in startino value is zero then we start with the first
inode in the AG that the user passes in; otherwise, we iterate only
within the same AG as the passed-in inode.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>