system-linux: set pending to 0 on ifindex found or error for if_get_master

system-linux: set pending to 0 on ifindex found or error for if_get_master

Fix Coverity Defect 1559194 reporting an Infinite loop when
data->pending is not set to 0 on finishing the callback.

While this is a false-positive report as ACK is always called, this is a
good time to optimize the code and make the callback exit early if we
find the ifindex for the DSA conduit.

Correctly set pending to 0 on ifindex found or error for if_get_master
valid netlink callback.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>