XfsFileStoreBackend: default to disabling extsize on xfs

This appears to be responsible for the deep scrub mismatches on some rbd
workloads.

Fixes: 8830
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #2103 from ceph/wip-8830

XfsFileStoreBackend: default to disabling extsize on xfs

Reviewed-by: Sage Weil <sage@redhat.com>