xfs: separate freelist fixing into a separate helper

Break up xfs_free_extent() into a helper that fixes the freelist.
This helper will be used subsequently to ensure the freelist during
deferred rmap processing.

[darrick: refactor to put this at the head of the patchset]

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>