dm kcopyd: fix job_pool leak

dm kcopyd: fix job_pool leak

Fix memory leak introduced by commit 6c7f58ebf5ba46f9406922974eed8e15eb4e1163
(dm snapshot: skip reading origin when overwriting complete chunk).

When allocating a set of jobs from kc->job_pool, job->master_job must be
set (to point to itself) so that the mempool item gets freed when the
master_job completes.

master_job was introduced by commit 52984fea524f6cbdbb270ffc85d94634f04ea121
(dm kcopyd: preallocate sub jobs to avoid deadlock)

Reported-by: Michael Leun <ml@newton.leun.net>
Cc: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>