libceph: simplify ceph_monc_handle_map()

ceph_monc_handle_map() confuses static checkers which report a
false use-after-free on monc->monmap, missing that monc->monmap and
client->monc.monmap is the same pointer.

Use monc->monmap consistently and get rid of "old", which is redundant.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>