mount.ceph: collect v2 addresses for non-legacy ms_mode options
The kclient recently had support for msgr2 merged, and with that we have
a new ms_mode mount option. The mount helper's support for autodiscovery
of mons currently always ignores v2 addresses.
Change the helper to look for the ms_mode in the provided options and if
it's set to anything but "legacy", have it collect v2 addresses instead
of v1 addrs. The default is still "legacy" so if the option is not
provided, we will still collect v1 addrs.
Fixes: https://tracker.ceph.com/issues/48765 Signed-off-by: Jeff Layton <jlayton@redhat.com>