xfs: reflow the tail end of xfs_rtallocate_extent_block

Change polarity of a check so that the successful case of being able to
allocate an extent is in the main path of the function and error handling
is on a 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>