krbd: add support for msgr2

krbd: add support for msgr2

Recognize ms_mode map option and filter initial monitor addresses
accordingly: if ms_mode is not given or ms_mode=legacy, discard v2
addresses, otherwise discard v1 addresses.

Note that nothing was discarded (i.e. v2 addresses were passed to
the kernel) previously.  The intent was to preserve that behaviour
in case ms_mode is not given, allowing to change the kernel default
in the future.  However, it turns out that mount.ceph helper has
been misguidedly discarding v2 addresses since commit eae01275134e
("mount.ceph: fork a child to get info from local configuration"),
so that ship has sailed.

Fixes: https://tracker.ceph.com/issues/48976
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 08f714964b7fe5024504818f01328a41acc24965)