fs: start to split up fs.h

Take first steps to split up fs.h. Add fs_super_types.h and fs_super.h
headers that contain the types and functions associated with super
blocks respectively.

To: linux-fsdevel@vger.kernel.org
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jan Kara <jack@suse.cz>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

--- b4-submit-tracking ---
# This section is used internally by b4 prep for tracking purposes.
{
  "series": {
    "revision": 1,
    "change-id": "20251104-work-fs-header-16f780a431c1",
    "prefixes": [],
    "base-branch": "master.v6.18-rc3"
  }
}