SUNRPC: Remove rpc_authflavor_lock in favour of RCU locking

Module removal is RCU safe by design, so we really have no need to
lock the auth_flavors[] array. Substitute a lockless scheme to
add/remove entries in the array, and then use rcu.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>