verity: use atomic_set instead of atomic_cmpxchg in verify_path

verity: use atomic_set instead of atomic_cmpxchg in verify_path

When moving from the READY to VERIFIED state it is sufficient to use
an atomic_set since we know the previous state MUST have been
either READY or VERIFIED.

BUG=9752
TEST=Ran tests in verity.git. Ran platform_DMVerityCorruption on H/W.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>;
kernel.git Review URL: http://codereview.chromium.org/6694010

TBRing. Already LGTMed and committed to kernel.git.

Change-Id: Ie2d57adf8752b2e4e9de9a53645156083382d03f

TBR=wad@chromium.org,taysom@chromium.org,ups@chromium.org

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=aacc804