mac80211: fix "warning: ‘target_metric’ may be used uninitialized"

This fixes:

net/mac80211/mesh_hwmp.c:603:26: warning: ‘target_metric’ may be used uninitialized in this function

target_metric is only consumed when reply = true so no bug exists here,
but not all versions of gcc realize it.  Initialize to 0 to remove the
warning.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>