Merge patch series "API for exporting connectable file handles to userspace"

Amir Goldstein <[email protected]> says:

These patches bring the NFS connectable file handles feature to
userspace servers.

They rely on Christian's and Aleksa's changes recently merged to v6.12.

The API I chose for encoding conenctable file handles is pretty
conventional (AT_HANDLE_CONNECTABLE).

open_by_handle_at(2) does not have AT_ flags argument, but also, I find
it more useful API that encoding a connectable file handle can mandate
the resolving of a connected fd, without having to opt-in for a
connected fd independently.

I chose to implemnent this by using upper bits in the handle type field
It may be that out-of-tree filesystems return a handle type with upper
bits set, but AFAIK, no in-tree filesystem does that.
I added some warnings just in case we encouter that.

I have written an fstest [1] and a man page draft [2] for the feature.

[1] https://github.com/amir73il/xfstests/commits/connectable-fh/
[2] https://github.com/amir73il/man-pages/commits/connectable-fh/

* patches from https://lore.kernel.org/r/20241011090023[email protected]:
  fs: open_by_handle_at() support for decoding "explicit connectable" file handles
  fs: name_to_handle_at() support for "explicit connectable" file handles
  fs: prepare for "explicit connectable" file handles

Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Christian Brauner <[email protected]>