io_uring: add mkdir and [sym]linkat support

This started out as an attempt to add mkdirat support to io_uring which
is heavily based on renameat() / unlinkat() support.

During the review process more operations were added (linkat, symlinkat,
mknodat) mainly to keep things uniform internally (in namei.c), and
with things changed in namei.c adding support for these operations to
io_uring is trivial, so that was done too (except for mknodat). See
https://lore.kernel.org/io-uring/20210514145259.wtl4xcsp5...

The first patch makes putname() ignore IS_ERR_OR_NULL names and converts
a couple of places where the check was already implemented in the
callers. No functional changes.

The second patch is preparation with no functional changes, makes
do_mkdirat accept struct filename pointer rather than the user string.

The third one leverages that to implement mkdirat in io_uring.

4-7 just convert other similar do_* functions in namei.c to accept
struct filename, for uniformity with do_mkdirat, do_renameat and
do_unlinkat. No functional changes there.

8 changes do_* helpers in namei.c to return ints rather than some of
them returning ints and some longs.

9-10 add symlinkat and linkat support to io_uring correspondingly.

Based on for-5.14/io_uring.

v7:
- rebase
- make putname() ignore IS_ERR_OR_NULL names, remove conditional calls
  to it from the callers

v6:

- rebase
- add safety checks for IOPOLL mode
- add safety checks for unused sqe parts
- drop mknodat support from io_uring as requested by Jens
- add Christian's Acked-by

v5:
- rebase
- add symlinkat, linkat and mknodat support to io_uring

v4:
- update do_mknodat, do_symlinkat and do_linkat to accept struct
  filename for uniformity with do_mkdirat, do_renameat and do_unlinkat;

v3:
- rebase;

v2:
- do not mess with struct filename's refcount in do_mkdirat, instead add
  and use __filename_create() that does not drop the name on success;

Dmitry Kadashev (10):
  namei: ignore ERR/NULL names in putname()
  fs: make do_mkdirat() take struct filename
  io_uring: add support for IORING_OP_MKDIRAT
  fs: make do_mknodat() take struct filename
  fs: make do_symlinkat() take struct filename
  namei: add getname_uflags()
  fs: make do_linkat() take struct filename
  fs: update do_*() helpers to return ints
  io_uring: add support for IORING_OP_SYMLINKAT
  io_uring: add support for IORING_OP_LINKAT

 fs/exec.c                     |   8 +-
 fs/internal.h                 |   8 +-
 fs/io_uring.c                 | 196 ++++++++++++++++++++++++++++++++++
 fs/namei.c                    | 139 +++++++++++++++---------
 include/linux/fs.h            |   1 +
 include/uapi/linux/io_uring.h |   4 +
 6 files changed, 300 insertions(+), 56 deletions(-)

-- 
2.30.2