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: NMikulas Patocka <mpatocka@redhat.com>
Signed-off-by: NMike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -282,6 +282,8 @@ static int persistent_memory_claim(struct dm_writecache *wc)
 			while (daa-- && i < p) {
 				pages[i++] = pfn_t_to_page(pfn);
 				pfn.val++;
+				if (!(i & 15))
+					cond_resched();
 			}
 		} while (i < p);
 		wc->memory_map = vmap(pages, p, VM_MAP, PAGE_KERNEL);