tc: ematch: Ignore all-zero mask value when printing filters

tc: ematch: Ignore all-zero mask value when printing filters

The optional mask which may be added to int values is considered by the
kernel only if it is non-zero, therefore tc should only then also print
it.

Without this, not passing a mask value like so:

| # tc filter add dev d0 parent 8001: \
| 	basic match meta\(vlan eq 1\) \
| 	classid 8001:1

Would lead to tc printing an all-zero mask later:

| # tc filter show dev d0
| filter parent 8001: protocol all pref 49151 basic
| filter parent 8001: protocol all pref 49151 basic handle 0x1 flowid 8001:1
|   meta(vlan mask 0x00000000 eq 1)

This is obviously confusing as an all-zero mask strictly means to
eliminate all bits from the value, but the opposite is the case.

Cc: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Phil Sutter <phil@nwl.cc>