mon: allow full flag to be manually cleared

This is like a temporary measure as the mon will try to set them again,
but we have run into cases where the mon was misbehaving (failing to clear
the flag) and we wanted to do it.  Note that the mon will likely set it
again on the next tick() anyway.

If we're going to clear it, we may as well be able to set it, too (again,
even if the mon is going to clear it soon).  If nothing else this is useful
for writing tests.

Fixes: #9323
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #3146 from ceph/wip-9323

mon: allow full flag to be manually cleared

Reviewed-by: Joao Eduardo Luis <joao@redhat.com>