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>