ceph: add selinux support

When creating new file/directory, use security_dentry_init_security() to
prepare selinux context for the new inode, then send openc/mkdir request
to MDS, together with selinux xattr.

security_dentry_init_security() only supports single security module and
only selinux has dentry_init_security hook. So only selinux is supported
for now. We can add support for other security modules once kernel has a
generic version of dentry_init_security()

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>