mds: fix calls to Objecter::wait_for_map

mds: fix calls to Objecter::wait_for_map

These were wrong in the earlier commit:
"mds: use lock-safe OSDMap accessors; adjust Objecter wait_for_map call"

Rather than checking epoch explicitly and dropping the lock before
calling wait_for_map, just make a single call to wait_for_map and handle
the return code to learn whether we are waiting or not.

Signed-off-by: John Spray <john.spray@redhat.com>