Bring back revision 1.122 with a fix preventing a use-after-free by

serializing calls to pipe_buffer_free(). Repeating the previous commit
message:

Instead of performing three distinct allocations per created pipe,
reduce it to a single one. Not only should this be more performant, it
also solves a kqueue related issue found by visa@ who also requested
this change: if you attach an EVFILT_WRITE filter to a pipe fd, the
knote gets added to the peer's klist. This is a problem for kqueue
because if you close the peer's fd, the knote is left in the list whose
head is about to be freed. knote_fdclose() is not able to clear the
knote because it is not registered with the peer's fd.
FreeBSD also takes a similar approach to pipe allocations.

once again ok mpi@ visa@

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