Merge patch series "OPENAT2_REGULAR flag support for openat2"

Dorjoy Chowdhury <dorjoychy111@gmail.com> says:

I came upon this "Ability to only open regular files" uapi feature
suggestion from
https://uapi-group.org/kernel-features/#ability-to-only-open-regular-files
and thought it would be something I could do as a first patch and get to
know the kernel code a bit better.

The following filesystems have been tested by building and booting the
kernel x86 bzImage in a Fedora 43 VM in QEMU. I have tested with
OPENAT2_REGULAR that regular files can be successfully opened and
non-regular files (directory, fifo etc) return -EFTYPE.

- btrfs
- NFS (loopback)
- SMB (loopback)

Christian Brauner (Amutable) <brauner@kernel.org>:

All atomic_open implementations were audited for OPENAT2_REGULAR handling.
Explicit checks were added to ceph, gfs2, nfs (v4), and cifs/smb — these
are the filesystems whose atomic_open can encounter an existing non-regular
file and would otherwise call finish_open() on it or return a misleading
error code. The checks allow these filesystems to return -EFTYPE directly
and avoid unnecessary open+close round-trips.

The remaining implementations (9p, fuse, vboxsf, nfs v2/v3) don't need
explicit checks. They only call finish_open() on freshly created files
(always S_IFREG) and use finish_no_open() for lookup hits, letting the
VFS catch non-regular files via the do_open() safety net. Notably, fuse
also validates the server response (S_ISREG check on the reply) before
reaching finish_open().

* patches from https://patch.msgid.link/20260328172314.45807-1-dorjoychy111@gmail.com:
  kselftest/openat2: test for OPENAT2_REGULAR flag
  openat2: new OPENAT2_REGULAR flag support

Link: https://patch.msgid.link/20260328172314.45807-1-dorjoychy111@gmail.com
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>