make splice more generic

This series makes splice(2) work in more cases:

  - pipe to pipe splicing (zero copy)
  - fallback splice_read which uses readv()
  - fallback splice_write which uses writev()

Just after cleaning up my patches after a vacation I found Max
Kellermann's patch on LKML implementing the first part.  I'll still
post mine, because it's slightly simpler (no ref + unref on the buffer
if not necessary).

One more generalization would be to allow splice to work on two
non-pipes, using an internal intermediate pipe, a-la do_splice_direct().

Comments?

Thanks,
Miklos
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