svcrpc: document lack of some memory barriers

We're missing memory barriers in net/sunrpc/svcsock.c in some spots we'd
expect them.  But it doesn't appear they're necessary in our case, and
this is likely a hot path--for now just document the odd behavior.

Kosuke Tatsukawa found this issue while looking through the linux source
code for places calling waitqueue_active() before wake_up*(), but
without preceding memory barriers, after sending a patch to fix a
similar issue in drivers/tty/n_tty.c  (Details about the original issue
can be found here: https://lkml.org/lkml/2015/9/28/849

).

Reported-by: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>