net/9p: remove unused p9_req_t aux field

The p9_req_t field 'aux' has not been used in a very long time,
remove leftover field declaration

Link: http://lkml.kernel.org/r/1580941152-12973-1-git-send-email-asmadeus@codewreck.org


Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>