dm writecache: allow allocations larger than 2GiB

The function kvmalloc_node limits the allocation size to INT_MAX. This
limit will be overflowed if dm-writecache attempts to map a device with
1TiB or larger length. This commit changes kvmalloc_array to vmalloc_array
to avoid the limit.

The commit also changes vmalloc(array_size()) to vmalloc_array().

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>