xfs: rework the rtalloc fallback handling

xfs: rework the rtalloc fallback handling

xfs_rtallocate currently has two fallbacks, when an allocation fails:

 1) drop the requested extent size alignment, if any, and retry
 2) ignore the locality hint

Oddly enough it does those in order, as trying a different location
is more in line with what the user asked for, and does it in a very
unstructured way.

Lift the fallback to try to allocate without the locality hint into
xfs_rtallocate to both perform them in a more sensible order and to
clean up the code.

Signed-off-by: Christoph Hellwig <hch@lst.de>