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

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

(This cherry-picks b4201cc4fc6e1c57d6d306b1f787865043d60129 upstream)

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.

Change-Id: I13923fda9d314f48196c29e4354133dfe01f5abd
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[jstultz: Cherry-picked to android-4.4]
Signed-off-by: John Stultz <john.stultz@linaro.org>