9p: Fix comment on smp_wmb

The previous comment misled me into thinking the barrier wasn't needed
at all.

Link: http://lkml.kernel.org/r/20180711210225.19730-2-willy@infradead.org


Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Greg Kurz <groug@kaod.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>