dm writecache: handle REQ_FUA

commit c1005322 upstream.

Call writecache_flush() on REQ_FUA in writecache_map().

Cc: stable@vger.kernel.org # 4.18+
Signed-off-by: Maged Mokhtar <mmokhtar@petasan.org>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>