msm: ipa: Add new IOCTLs to get and set MTU

Add a new IOCTL that will allow userspace modules to
get and set the v4/v6 mtu for a specific rmnet interface
for IPA driver and IPACM to use.

Change-Id: Iaf0685ef7095960ddd1cd3fff803956a18e0a942
Signed-off-by: Chaitanya Pratapa <cpratapa@codeaurora.org>
Signed-off-by: Michael Adisumarta <madisuma@codeaurora.org>