net: net_test: add tests for IP tunnel flags conversion helpers

Now that there are helpers for converting IP tunnel flags between the
old __be16 format and the bitmap format, make sure they work as expected
by adding a couple of tests to the networking testing suite. The helpers
are all inline, so no dependencies on the related CONFIG_* (or a
standalone module) are needed.

Cover three possible cases:

1. No bits past BIT(15) are set, VTI/SIT bits are not set. This
   conversion is almost a direct assignment.
2. No bits past BIT(15) are set, but VTI/SIT bit is set. During the
   conversion, it must be transformed into BIT(16) in the bitmap,
   but still compatible with the __be16 format.
3. The bitmap has bits past BIT(15) set (not the VTI/SIT one). The
   result will be truncated.
   Note that currently __IP_TUNNEL_FLAG_NUM is 17 (incl. special),
   which means that the result of this case is currently
   semi-false-positive. When BIT(17) is finally here, it will be
   adjusted accordingly.

Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>