osd: fix load_pgs handling of pg dirs without a head

If there is a pgid that passes coll_t::is_pg() but there is no head, we
will populate the pgs map but then fail later when we try to do
read_state.  This is a side-effect of 55f8579.

Take explicit note of _head collections we see, and then warn when we
find stray snap collections.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: fix load_pgs collection handling

On a _TEMP pg, is_pg() would succeed, which meant we weren't actually
hitting the cleanup checks.  Instead, restructure this loop as positive
checks and handle each type of collection we understand.

This fixes _TEMP cleanup.

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