ZWatcher says it does not need to lock wlinkMu because it is already
holding zheadMu and setupWatch runs with zheadMu locked. That is indeed
true, but the mistake here is that it i not only setupWatch that makes
access to wlinkTab. For example WatchNode.Open registers new entries
there only under wlinkMu:

https://lab.nexedi.com/nexedi/wendelin.core/-/blob/wendelin.core-2.0.alpha3-26-g79e6f7b9/wcfs/wcfs.go#L1819-1822

-> Fix it by always using wlinkMu when accessing wlinkTab.

My mistake from 6f0cdaff (wcfs: Provide isolation to clients)

Test is, hopefully, TODO.

/reviewed-by @levin.zimmermann
/reviewed-on nexedi/wendelin.core!18