mon: OSDMonitor: only thrash and propose if we are the leader

'thrash_map' is only set if we are the leader, so we would thrash and
propose the pending value if we are the leader.  However, we should keep
the 'is_leader()' check not only for clarity's sake (an unfamiliar reader
may cry OMGBUG, prompting to a patch much like this), but also because
we may lose a subsequent election and become a peon instead, while still
holding a 'thrash_map' value > 0 -- and we really don't want to propose
while being a peon.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>