qa/suites/rbd: place cache file on tmpfs for xfstests

qa/suites/rbd: place cache file on tmpfs for xfstests

The RWL mode needs DAX and is dog slow otherwise -- qemu_xfstests.yaml
job always hits the 6 hour max_job_time limit.

As our tmpfs instance is limited and qemu_xfstests.yaml opens three
images at the same time, reduce the "big cache" size to 5G.  This facet
was added to iron out 32-bit head/tail pointer issues and 5G still does
the job there.

Going through the loop device is needed because tmpfs doesn't support
O_DIRECT.

Fixes: https://tracker.ceph.com/issues/55400
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 23759e003435499105d1076875fdef2ca7e55a80)