dm cache metadata: ignore hints array being too small during resize

Commit fd2fa954 ("dm cache metadata: save in-core policy_hint_size to
on-disk superblock") enabled previously written policy hints to be
used after a cache is reactivated.  But in doing so the cache
metadata's hint array was left exposed to out of bounds access because
on resize the metadata's on-disk hint array wasn't ever extended.

Fix this by ignoring that there are no on-disk hints associated with the
newly added cache blocks.  An expanded on-disk hint array is later
rewritten upon the next clean shutdown of the cache.

Fixes: fd2fa954 ("dm cache metadata: save in-core policy_hint_size to on-disk superblock")
Cc: stable@vger.kernel.org
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>