xfs: simplify the xfs_bmap_btree_to_extents calling conventions

xfs: simplify the xfs_bmap_btree_to_extents calling conventions

Source kernel commit: b101e3342a34404f2cc2daaad569afcae68452b0

Move boilerplate code from the callers into xfs_bmap_btree_to_extents:

- exit early without failure if we don't need to convert to the
extent format
- assert that we have a btree cursor
- don't reinitialize the passed in logflags argument

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
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>