udevd: use a tighter loop for compare_devpath()

This crops up in my threaded udevd profiles from time to time.
It's not consistent - probably due to variations in the number
of concurrent events - but it can hit 4% user time and higher.

The change halves the user time spent in compare_devpath().

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>