open/accept directly into io_uring fixed file table

Implement an old idea allowing open/accept io_uring requests to register
a newly created file as a io_uring's fixed file instead of placing it
into a task's file table. The switching is encoded in io_uring's SQEs
by setting sqe->buf_index/file_index, so restricted to 2^16-1. Don't
think we need more, but may be a good idea to scrap u32 somewhere
instead.

From the net side only needs a function doing __sys_accept4_file()
but not installing fd, see 2/4.

Only RFC for now, the new functionality is tested only for open yet.
I hope we can remember the author of the idea to add attribution.

Pavel Begunkov (4):
  io_uring: allow open directly into fixed fd table
  net: add an accept helper not installing fd
  io_uring: hand code io_accept()' fd installing
  io_uring: accept directly into fixed file table

 fs/io_uring.c                 | 113 +++++++++++++++++++++++++++++-----
 include/linux/socket.h        |   3 +
 include/uapi/linux/io_uring.h |   2 +
 net/socket.c                  |  71 +++++++++++----------
 4 files changed, 138 insertions(+), 51 deletions(-)

-- 
2.32.0