msm: ipa: add null terminator

Add null terminator at the end of string
extend_ioctl_data.u.rmnet_mux_val.vchannel_name
to avoid potential security issue.

Change-Id: I57fe3a9f7e3ad6a499b62a9cfc49bc6b2f3b42e0
Acked-by: Shihuan Liu <shihuanl@qti.qualcomm.com>
Signed-off-by: Skylar Chang <chiaweic@codeaurora.org>