osd: fix load_pgs handling of pg dirs without a head

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>
Reviewed-by: Samuel Just <sam.just@inktank.com>