xfs: consider minlen sized extents in xfs_rtallocate_extent_block

minlen is the lower bound on the extent length that the caller can
accept, and maxlen is at this point the maximal available length.
This means a minlen extent is perfectly fine to use, so do it.  This
matches the equivalent logic in xfs_rtallocate_extent_exact that also
accepts a minlen sized extent.

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>