9p: acl: fix uninitialized iattr access

iattr is passed to v9fs_vfs_setattr_dotl which does send various
values from iattr over the wire, even if it tells the server to
only look at iattr.ia_valid fields this could leak some stack data.

Link: http://lkml.kernel.org/r/1536339057-21974-2-git-send-email-asmadeus@codewreck.org
Addresses-Coverity-ID: 1195601 ("Uninitalized scalar variable")
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>