FWIW, I'm not sure if skb_splice_bits() can't land us in trouble; fragments
might come from compound pages and I'm not entirely convinced that we won't
end up with coalesced fragments putting more than PAGE_SIZE into a single
pipe_buffer.  And that could badly confuse a bunch of code.

Can that legitimately happen?  If so, we'll need to audit quite a few
->splice_write()-related codepaths; FUSE, in particular, is very likely
to be unhappy with that kind of stuff, and it's not the only place where
we might count upon never seeing e.g. longer than PAGE_SIZE chunks in
bio_vec.  It shouldn't be all that hard to fix, but if the whole thing
is simply impossible, I would rather avoid that round of RTFS at the moment...

Comments?