dm verity: fix crash on bufio buffer that was allocated with vmalloc

Since commit d1ac3ff0 ("dm verity: switch to using asynchronous hash
crypto API") dm-verity uses asynchronous crypto calls for verification,
so that it can use hardware with asynchronous processing of crypto
operations.

These asynchronous calls don't support vmalloc memory, but the buffer data
can be allocated with vmalloc if dm-bufio is short of memory and uses a
reserved buffer that was preallocated in dm_bufio_client_create().

Fix verity_hash_update() so that it deals with vmalloc'd memory
correctly.

Reported-by: "Xiao, Jin" <jin.xiao@intel.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Fixes: d1ac3ff0 ("dm verity: switch to using asynchronous hash crypto API")
Cc: stable@vger.kernel.org # 4.12+
Signed-off-by: Mike Snitzer <snitzer@redhat.com>