dm writecache: fix smatch warning about invalid return from writecache_map

dm writecache: fix smatch warning about invalid return from writecache_map

There's a smatch warning "inconsistent returns '&wc->lock'" in
dm-writecache. The reason for the warning is that writecache_map()
doesn't drop the lock on the impossible path.

Fix this warning by adding wc_unlock() after the BUG statement (so
that it will be compiled-away anyway).

Fixes: df699cc16ea5e ("dm writecache: report invalid return from writecache_map helpers")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>