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>
(cherry picked from commit 01cd3cdc726a3e838bce05b355a021778b4e5db1)