mgr/volumes: Fix subvolume creation in FIPS enabled system.

mgr/volumes: Fix subvolume creation in FIPS enabled system.

The md5 checksum is used in the construction of legacy
subvolume config filename. It's not used for security reason.
Hence marking the 'usedforsecurity' flag to false to
make it FIPs compliant.

The usage of md5 was always in there. The commit 373a04cf734
made it to get exercised in 'open_subvol' which is pre-requisite
for all the subvolume operations and hence subvolume
creation has failed.

Fixes: https://tracker.ceph.com/issues/56727
Signed-off-by: Kotresh HR <khiremat@redhat.com>