net: tc_skbedit: Factor a helper out of is_tcf_skbedit_{mark, ptype}()

The two functions is_tcf_skbedit_mark() and is_tcf_skbedit_ptype() have a
very similar structure. A follow-up patch will add one more such function.
Instead of more cut'n'pasting, extract a helper function that checks
whether a TC action is an skbedit with the required flag. Convert the two
existing functions into thin wrappers around the helper.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>