[CORE] vmm_blockdev: Minor fix in submit_request() and abort_request()

We typically clear 'bdev' field of vmm_request upon request completion
or request failure. In other words, 'bdev' field of vmm_request is not
NULL only when vmm_request is queued to some request queue.

Now if we have a block devices (such as RBD block device) that completes
request in make_request() itself then for SMP host current submit_request()
hangs after returning from make_request() because complete_request() clears
'bdev' field of vmm_request.

This patch fixes submit_request() and abort_request() for above described
issue.

Signed-off-by: Anup Patel <anup@brainfault.org>