ReplicatedPG::maybe_handle_cache: do not promote before checking full

ReplicatedPG::maybe_handle_cache: do not promote before checking full

Otherwise, ops with need_promote() can order past ops without it
causing an rgw write sequence like:

writefull 0~0
writefull 0~512kb
write 512kb~512kb

to reorder to

writefull 0~0 -- delayed: full
writefull 0~512kb -- delayed: full
write 512kb~512kb -- promote and execute
writefull 0~0 -- execute
writefull 0~512kb -- execute

Fixes: 13240
Signed-off-by: Samuel Just <sjust@redhat.com>