selinux: Implement the new mount API LSM hooks

selinux: Implement the new mount API LSM hooks

Implement the new mount API LSM hooks for SELinux.  At some point the old
hooks will need to be removed.

Question: Should the ->fs_context_parse_source() hook be implemented to
check the labels on any source devices specified?

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paul Moore <paul@paul-moore.com>
cc: Stephen Smalley <sds@tycho.nsa.gov>
cc: selinux@tycho.nsa.gov
cc: linux-security-module@vger.kernel.org