rmnet: Reduce synchronize_rcu calls

Avoid callig synchronize_rcu() in a loop for every rmnet vnd
during SSR or shutdown by doing it only once.

Change-Id: I9788a431ca227fc49eee24d94d0803354d10fbc6
Acked-by: Weiyi Chen <weiyic@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>