Add support for vectored registered buffers

Add registered buffer support for vectored io_uring operations. That
allows to pass an iovec, all entries of which must belong to and
point into the same registered buffer specified by sqe->buf_index.

The series covers zerocopy sendmsg and reads / writes. Reads and
writes are implemented as new opcodes, while zerocopy sendmsg
reuses IORING_RECVSEND_FIXED_BUF for the user API.

Results are aligned to what one would expect from registered buffers:

t/io_uring + nullblk, single segment 16K:
  34 -> 46 GiB/s
examples/send-zerocopy.c default send size (64KB):
  82558 -> 123855 MB/s

The series is placed on top of 6.15 + zcrx + epoll.

v4: Address cast and include warnings
v3: Handle 32 bit where bvec is larger than iovec
v2: Nowarn alloc
    Cap bvec caching
    Check length overflow
    Reject 0 len segments
    Check op direction
    Other minor changes

Pavel Begunkov (9):
  io_uring: introduce struct iou_vec
  io_uring: add infra for importing vectored reg buffers
  io_uring/rw: implement vectored registered rw
  io_uring/rw: defer reg buf vec import
  io_uring/net: combine msghdr copy
  io_uring/net: pull vec alloc out of msghdr import
  io_uring/net: convert to struct iou_vec
  io_uring/net: implement vectored reg bufs for zctx
  io_uring: cap cached iovec/bvec size

 include/linux/io_uring_types.h |  11 ++
 include/uapi/linux/io_uring.h  |   2 +
 io_uring/alloc_cache.h         |   9 --
 io_uring/net.c                 | 180 +++++++++++++++++++++------------
 io_uring/net.h                 |   6 +-
 io_uring/opdef.c               |  39 +++++++
 io_uring/rsrc.c                | 137 +++++++++++++++++++++++++
 io_uring/rsrc.h                |  23 +++++
 io_uring/rw.c                  |  99 ++++++++++++++++--
 io_uring/rw.h                  |   6 +-
 10 files changed, 420 insertions(+), 92 deletions(-)

-- 
2.48.1