net: qualcomm: rmnet: lock descriptor pool

In CPU hotplug scenarios, it is possible for the descriptor pool to
be accessed by multiple cores at the same time, leading to race conditions
where data can be maniuplated at the same time. This can leave the list
in an invalid state, leading to errors later in processing.

Change-Id: Id7652c47abf80f73f36a5bf89136f0e695887be1
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>