9p: roll p9_tag_remove into p9_req_put

mempool prep commit removed the awkward kref usage which didn't
allow passing client pointer easily with the ref, so we no longer
need a separate function to remove the tag from idr.

This has the side benefit that it should be more robust in detecting
leaks: umount will now properly catch unfreed requests as they still
will be in the idr until the last ref is dropped

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Dominique Martinet <[email protected]>
Reviewed-by: Christian Schoenebeck <[email protected]>