tun: Set CHECKSUM_UNNECESSARY if userspace passes this indication

Userspace may already know that the checksum validation is already
completed prior to being passed to the TUN interface. As a result,
computing checksum again in network stack may be a redundant
operation.

Add support to read this information from the TUN header flags
to skip checksum validation for these packets only.

This is useful in cases where the packet checksum was computed by
hardware for IPv4 / IPv6 TCP / UDP packets. Since the packet
intergrity was already verified for packets over the wire, subsequent
validation within the network stack is redundant work.

CRs-Fixed: 2333588
Change-Id: I18ee3408c05910207b205d6205f282e6f3599156
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>