Merge patch series "fs: allow statmount to fetch the fs_subtype and sb_source"

Jeff Layton <jlayton@kernel.org> says:

Meta has some internal logging that scrapes /proc/self/mountinfo today.
I'd like to convert it to use listmount()/statmount(), so we can do a
better job of monitoring with containers. We're missing some fields
though. This patchset adds them.

* patches from https://lore.kernel.org/r/20241111-statmount-v4-0-2eaf35d07a80@kernel.org:
  fs: add the ability for statmount() to report the sb_source
  fs: add the ability for statmount() to report the fs_subtype
  fs: don't let statmount return empty strings

Link: https://lore.kernel.org/r/20241111-statmount-v4-0-2eaf35d07a80@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>