mon/OSDMonitor: Ensure kvmon() is writeable before handling "osd new" cmd

mon/OSDMonitor: Ensure kvmon() is writeable before handling "osd new" cmd

Before proceeding to handle "osd new" mon command as part of
OSDMonitor::prepare_command_impl(), a check is made to verify if the
authmon is writeable. Later on, prepare_command_osd_new() invokes
KVMonitor::do_osd_new() to create pending dmcrypt keys and calls
propose_pending(). The propose could fail (with an assertion failure)
if there was a prior mon command that resulted in the kvmon invoking
propose_pending().

In order to avoid such a situation, introduce a check to verify that
kvmon is also writeable in OSDMonitor::prepare_command_impl(). If it
is not writeable, the op is pushed into the wait_for_active context
queue to be retried later.

Fixes: https://tracker.ceph.com/issues/55773
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>