net: tun: Implement TUN_NOCHECKSUM

Revert the changes to TUN_NOCHECKSUM from commit 88255375
("net: tun: convert to hw_features"). This is needed to handle
scenarios where GRO coalesced packets are passed on to userspace
with an invalid checksum to programs capturing packets using
packet sockets.

If the checksum was validated before GRO coalesced the packets,
it is unnecessary to validate the checksum again. Userspace
should be allowed to set TUN_NOCHECKSUM on the underlying tun
interface.

Change-Id: I96071e8a07a01efc71612d5a6979d493339e2735
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>