kernel: fix commit hash matching in need_to_install()

We don't need to install a new kernel if the "want" (version) commit
hash and the "have" (cur_sha1) commit hash match but the former is
shorter than the latter.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>
Merge pull request #376 from ceph/wip-sha-compare-fix

kernel: fix commit hash matching in need_to_install()