Commit 05325552 ('Support "ip link add help" for rtnl_link API') added a
check for specified help parameter. Though due to the place where it has
been added to, it is not possible anymore to force a given parameter to
be interpreted as interface name by prefixing it with 'dev '. Fix this
by forcing whatever follows 'dev' to be presumed as interface name.
Signed-off-by: Phil Sutter <phil@nwl.cc>