io_uring: add support for accept(4)

This series adds support for applications doing async accept()
through io_uring.

Patches 1+2 are just a prep patches, adding support for inheriting a
process file table for commands.

Patch 3 abstracts out __sys_accept4_file(), which is the same as
__sys_accept4(), except it takes a struct file and extra file flags.
Should not have any functional changes.

And finally patch 4 adds support for IORING_OP_ACCEPT. sqe->fd is
the file descriptor, sqe->addr holds a pointer to struct sockaddr,
sqe->addr2 holds a pointer to socklen_t, and finally sqe->accept_flags
holds the flags for accept(4).

The series is against my for-5.5/io_uring-wq tree, and also exists
as a for-5.5/io_uring-test branch. liburing has a basic test case for
this (test/accept.c).

Changes since v1:
- Respin on top of for-5.5/io_uring-wq branch, as io-wq is needed to
  support cancellation of requests.
- Rework files_struct inheritance to avoid cyclic dependencies. Thanks
  to Jann Horn for some great suggestions there.


 fs/io-wq.c                    |  14 +++
 fs/io-wq.h                    |   3 +
 fs/io_uring.c                 | 157 ++++++++++++++++++++++++++++++++--
 include/linux/socket.h        |   3 +
 include/uapi/linux/io_uring.h |   7 +-
 net/socket.c                  |  65 ++++++++------
 6 files changed, 217 insertions(+), 32 deletions(-)

-- 
Jens Axboe