Merge patch series "fhandle, pidfs: allow open_by_handle_at() purely based on file handle"

Christian Brauner <brauner@kernel.org> says:

Various filesystems such as pidfs and drm support opening file handles
without having to require a file descriptor to identify the filesystem.
The filesystem are global single instances and can be trivially
identified solely on the information encoded in the file handle.

This makes it possible to not have to keep or acquire a sentinal file
descriptor just to pass it to open_by_handle_at() to identify the
filesystem. That's especially useful when such sentinel file descriptor
cannot or should not be acquired.

For pidfs this means a file handle can function as full replacement for
storing a pid in a file. Instead a file handle can be stored and
reopened purely based on the file handle.

Such autonomous file handles can be opened with or without specifying a
a file descriptor. If no proper file descriptor is used the
FD_PIDFS_ROOT sentinel must be passed. This allows us to define further
special negative fd sentinels in the future.

Userspace can trivially test for support by trying to open the file
handle with an invalid file descriptor.

* patches from https://lore.kernel.org/20250624-work-pidfs-fhandle-v2-0-d02a04858fe3@kernel.org:
  selftests/pidfd: decode pidfd file handles withou having to specify an fd
  fhandle, pidfs: support open_by_handle_at() purely based on file handle
  uapi/fcntl: add FD_PIDFS_ROOT
  uapi/fcntl: add FD_INVALID
  uapi/fcntl: mark range as reserved
  fhandle: reflow get_path_anchor()
  pidfs: add pidfs_root_path() helper
  fhandle: rename to get_path_anchor()
  fhandle: hoist copy_from_user() above get_path_from_fd()
  fhandle: raise FILEID_IS_DIR in handle_type

Link: https://lore.kernel.org/20250624-work-pidfs-fhandle-v2-0-d02a04858fe3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

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