In the other repo, we have a few cases where we call
MatchesFromFilterRules with a single tailcfg.FilterRule. Rather than
allocating a slice and then looping over the single-item slice, split
the internal logic out to a function that takes a single FilterRule so
we can use that.
Updates tailscale/corp#26353
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I95f2a7ee9de130712497259941264f06f3d00777