9p: Do a couple of cleanups

Do a couple of cleanups to 9p:

 (1) Remove a couple of unused variables.

 (2) Turn a BUG_ON() into a warning, consolidate with another warning and
     make the warning message include the inode number rather than
     whatever's in i_private (which will get hashed anyway).

Suggested-by: Dominique Martinet <asmadeus@codewreck.org>
Link: https://lore.kernel.org/r/ZZULNQAZ0n0WQv7p@codewreck.org/


Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Dominique Martinet <asmadeus@codewreck.org>
cc: Eric Van Hensbergen <ericvh@kernel.org>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: v9fs@lists.linux.dev
cc: linux-cachefs@redhat.com
cc: linux-fsdevel@vger.kernel.org