splice, net: Fix splice_to_socket() to handle pipe bufs larger than a page

splice_to_socket() assumes that a pipe_buffer won't hold more than a single
page of data - but this assumption can be violated by skb_splice_bits()
when it splices from a socket into a pipe.

The problem is that splice_to_socket() doesn't advance the pipe_buffer
length and offset when transcribing from the pipe buf into a bio_vec, so if
the buf is >PAGE_SIZE, it keeps repeating the same initial chunk and
doesn't advance the tail index.  It then subtracts this from "remain" and
overcounts the amount of data to be sent.

The cleanup phase then tries to overclean the pipe, hits an unused pipe buf
and a NULL-pointer dereference occurs.

Fix this by not restricting the bio_vec size to PAGE_SIZE and instead
transcribing the entirety of each pipe_buffer into a single bio_vec and
advancing the tail index if remain hasn't hit zero yet.

Large bio_vecs will then be split up by iterator functions such as
iov_iter_extract_pages().

This result...