xfs: tidy up xfs_rtallocate_extent_exact

Use common code for both xfs_rtallocate_range calls by moving
the !isfree logic into the non-default branch.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>