udev: make algorithm that selects highest priority devlink less susceptible to race...

udev: make algorithm that selects highest priority devlink less susceptible to race conditions

Previously it was very likely, when multiple contenders for the symlink
appear in parallel, that algorithm would select wrong symlink (i.e. one
with lower-priority).

Now the algorithm is much more defensive and when we detect change in
set of contenders for the symlink we reevaluate the selection. Same
happens when new symlink replaces already existing symlink that points
to different device node.