kernel - Fix deep stack recursion in unix domain socket garbage collection

kernel - Fix deep stack recursion in unix domain socket garbage collection

* A chain of unix domain sockets with the previous fd passed into the
  next one can cause a kernel stack overflow.

* Flatten the portion of the gc which would otherwise recurse by placing
  the fp on a list instead of calling a nested closef(), then running
  the closes at the top level.

Reported-by: Venkatesh Srinivas <me@endeavour.zapto.org>