U9fs(4) misuses Fcall.afid in its p9any authentication module.

The afid field of Fcall structure is only valid with Tauth or Tattach.
Tread, Twrite, Tclunk should use rx->fid instead.  It's been lucky so
far to get the job done because rx->afid survives from previous
Tauth/Tattach.  The issue pops up when several authentications happen
concurrently.

Test case:
	u9fs = 'host with u9fs on tcp!*!564'
	9fs $u9fs; for(i in 1 2 3 4 5 6 7 8){ 9fs $u9fs & }; wait
Should:
	no errors
Should not:
	mount failed: authentication failed