selftests: ublk: move common code into common.c

Move two functions for initializing & de-initializing backing file
into common.c.

Also move one common helper into kublk.h.

Prepare for supporting ublk-stripe.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322093218.431419-5-ming.lei@redhat.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>