rmnet_core: fix race condition in rmnet_get_packets

In rmnet powersave work, rmnet_get_packets() could access NULL dev pointer
if rmnet_dellink() is nullifying the dev pointer at the same time.

18377 [   72.651710][ T1527] Unable to handle kernel NULL pointer dereference
at virtual address 00000000000009d0

18424 [   72.653999][ T1527] Call trace:
18425 [   72.654085][ T1527]  rmnet_get_packets+0xc4/0x11c [rmnet_core]
18426 [   72.654170][ T1527]  qmi_rmnet_check_stats_2+0x80/0x410 [rmnet_core]
18427 [   72.654180][ T1527]  process_one_work+0x260/0x804

This change Uses the rcu variant of the hlist traversal function in
rmnet_get_packet for safe concurrency with the hlist del primitives.
It also checks dev pointer before accessing the dev private structure.
The existing synchronize rcu call in rmnet_dellink ensures that the ep
and dev structure are not freed while being referenced in rcu read
session of rmnet_get_packets.

Change-Id: Ib5f5aff6e76f9fffd9110a2aa924ad6ab090991f
Signed-off-by: Weiyi Chen <quic_weiyic@quicinc.com>