dm writecache: fix unnecessary NULL check warnings

Remove NULL checks before vfree() to fix these warnings:
./drivers/md/dm-writecache.c:2008:2-7: WARNING: NULL check before some
freeing functions is not needed.
./drivers/md/dm-writecache.c:2024:2-7: WARNING: NULL check before some
freeing functions is not needed.

Signed-off-by: Tian Tao <tiantao6@hisilicon.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>