xfs: retry allocations when locality-based search fails

If a realtime allocation fails because we can't find a sufficiently
large free extent satisfying locality rules, relax the locality rules
and try again.  This reduces the occurrence of short writes to realtime
files when the write size is large and the free space is fragmented.

This was originally discovered by running generic/186 with the realtime
reflink patchset and a 128k cow extent size hint, but the short write
symptoms can manifest with a 128k extent size hint and no reflink, so
apply the fix now.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>