dm writecache: add cond_resched to loop in persistent_memory_claim()

Add cond_resched() to a loop that fills in the mapper memory area
because the loop can be executed many times.

Fixes: 48debafe ("dm: add writecache target")
Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>