ifenslave: Fix unused variable warnings.

Documentation/networking/ifenslave.c: In function ‘if_getconfig’:
Documentation/networking/ifenslave.c:508:14: warning: variable ‘mtu’ set but not used [-Wunused-but-set-variable]
Documentation/networking/ifenslave.c:508:6: warning: variable ‘metric’ set but not used [-Wunused-but-set-variable]

The purpose of this function is to simply print out the values
it probes, so...

Signed-off-by: David S. Miller <davem@davemloft.net>