soc: qmi: dfc poison overwritten fix

Fix use-after-free due to access of qmi shared memory by dfc client
after the free.

A race condition can happen after qmi null pointer check but before
its variables get updated if functions run in different thread from
the change_link that could free the qmi memory anytime. Add the
rtnl_lock to protect the qmi get and access since qmi allocate/free
is also under rtnl_lock.

CRs-Fixed: 2326554
Change-Id: Ib65025e02e90edf7e2035ccb47c8c0da882b8c40
Acked-by: Ning Cai <ncai@qti.qualcomm.com>
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>