xfs: invert a check in xfs_rtallocate_extent_block

Doing a break in the else side of a conditional is rather silly.  Invert
the check, break ASAP and unindent the other leg.

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>