mon/OSDMonitor: search for latest full osdmap if record version is missing

In 97462a3213e5e15812c79afc0f54d697b6c498b1 we tried to search for a
recent full osdmap but were looking at the wrong key.  If full_0 was
present we could record that the latest full map was last_committed even
though it wasn't present.  This is fixed in 76cd7ac1c, but we need to
compensate for when get_version_latest_full() gives us a back version
number by repeating the search.

Fixes: #5737
Signed-off-by: Sage Weil <sage@inktank.com>