sunrpc: document locking rules for svc_exit_thread()

The locking required for svc_exit_thread() is not obvious, so document
it in a kdoc comment.

Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>