msm: ipa: Fix the mutex lock

Anytime before getting a chance to modify NAT pdn,
the NAT memory could be freed while waiting in lock.
Move the mutex lock to appropriate place.
And validate the NAT mem and pdn index after taking
mutex lock.

Change-Id: I7b3c39537be142b60c5ea87c37a80a04d94e8608
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>