CHROMIUM: dm: verity: fix warnings

CHROMIUM: dm: verity: fix warnings

Fixes:

drivers/md/dm-verity.c:109:1: warning: return from incompatible pointer type [enabled by default]
drivers/md/dm-verity.c:81:1: warning: return from incompatible pointer type [enabled by default]
drivers/md/dm-verity.c:85:1: warning: return from incompatible pointer type [enabled by default]

BUG=chromium-os:34287
TEST=build, check lack of warnings.

Change-Id: Iaff319f774a152e179836cb765ce951d97dbca9f
Signed-off-by: Olof Johansson <olofj@chromium.org>
Reviewed-on: https://gerrit.chromium.org/gerrit/32828
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>