Fix potential scheduling while atomic in mesh_path_add.

Calling synchronize_rcu() under write-lock-ed pathtbl_resize_lock may
result in this warning (and other side effects).

It looks safe just dropping this lock before calling synchronize_rcu.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>