msm: ipa: fix race condition around sys->len

Changes to gaurd sys->len with a spinlock to
avoid race condition between incrementing and
decrementing this value.

Change-Id: I638fea5a1a9cdda10b471fb4fa500079024e24c7
CRs-Fixed: 2093851
Acked-by: Jyothi Jayanthi <jyothij@qti.qualcomm.com>
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>