xfs: create agfl block free helper function

xfs: create agfl block free helper function

Source kernel commit: 4223f659dd3edd9e561d90488c6ae332a0a05148

Refactor the AGFL block free code into a new helper such that it can
be invoked from deferred context. No functional changes.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>