ReplicatedPG: fix CEPH_OSD_OP_CREATE on cache pools

The following

./ceph osd pool create data-cache 8 8
./ceph osd tier add data data-cache
./ceph osd tier cache-mode data-cache writeback
./ceph osd tier set-overlay data data-cache

./rados -p data create foo
./rados -p data stat foo

results in

  error stat-ing data/foo: No such file or directory

even though foo exists in the data-cache pool, as it should.  STAT
checks for (exists && !is_whiteout()), but the whiteout flag isn't
cleared on CREATE as it is on WRITE and WRITEFULL.  The problem is
that, for newly created 0-sized cache pool objects, CREATE handler in
do_osd_ops() doesn't get a chance to queue OP_TOUCH, and so the logic
in prepare_transaction() considers CREATE to be a read and therefore
doesn't clear whiteout.  Fix it by allowing CREATE handler to queue
OP_TOUCH at all times, mimicking WRITE and WRITEFULL behaviour.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>