fs: document semantics of kstat::{uid,gid} fields

The uid stored in struct kstat is logically a vfsuid; file systems
initialize it by converting a kuid (filesystem perspective) to a vfsuid
(mount perspective), then use vfsuid_into_kuid(), which essentially just
typecasts from vfsuid to kuid.

For now, just add a comment to note this mismatch between C type and
semantic type.

Below are some notes for anyone who wants to refactor this in the future.

There are probably two options to refactor this away:

1. Change the type of kstat::uid to vfsuid_t, and perform the conversion
   from vfsuid to userspace-uid in the VFS layer. This wouldn't change
   machine code, just be more semantically correct.
2. Change the semantics of kstat::uid to really be a kuid_t, and let the
   VFS layer take care of doing the translation from kuid to vfsuid that is
   currently done in filesystem code (or in generic_fillattr, on behalf of
   the filesystem code).

Option 2 is probably neater since it moves more logic into the generic VFS
layer, and this is something that is expected to work the same way in all
file systems?

The following coccinelle script:
```
virtual context

@@
struct kstat *stat;
@@
* stat->uid

@@
struct kstat *stat;
@@
* stat->gid

@@
struct kstat stat;
@@
* stat.uid

@@
struct kstat stat;
@@
* stat.gid
```
detects 43 field accesses to these uid/gid fields.

Signed-off-by: Jann Horn <jannh@google.com>
Link: https://patch.msgid.link/20260803-vfs-comment-stat-uid-v1-1-162d062b737c@google.com
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

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