ceph: use separate class for ceph sockets' sk_lock
Use a separate class for ceph sockets to prevent lockdep confusion.
Because ceph sockets only get passed kernel pointers, there is no
dependency from sk_lock -> mmap_sem. If we share the same class as other
sockets, lockdep detects a circular dependency from
because dependencies are noted from both ceph and user contexts. Using
a separate class prevents the sk_lock(ceph) -> mmap_sem dependency and
makes lockdep happy.