mds: rebuild snaprealm cache if last_modified or change_attr changed

mds: rebuild snaprealm cache if last_modified or change_attr changed

For the	server side snapdir visibility changes to be transported to the
client — SnapRealm cache needs to be rebuilt otherwise the same metadata
would be sent via the send_snap_update() in C_MDS_inode_update_finish() while
setting the `ceph.dir.subvolume.snaps.visible` vxattr.

The condition used to check for the `seq` and `last_destroyed` against their
cached values but for the vxattr change, it's a rather non-feasible
heavylifting to update the `seq` which involves a set of steps to prepare the
op, commit the op, journal the changes and update snap-server/client(s) just
for a mere flag update (and updating last_destroyed anyway doesn't make sense
for this case). So, compare last_modified and change_attr with their cached
values to check if the SnapRealm cache should be rebuilt. These values are
incremented in the Server::handle_client_setvxattr while toggling the
snapshot visibility xattr and this would enforce a cache rebuild.

Fixes: https://tracker.ceph.com/issues/71740
Signed-off-by: Dhairya Parmar <dparmar@redhat.com>