svcrpc: document lack of some memory barriers

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.

I found this issue when I was 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).

Signed-off-by: Kosuke Tatsukawa <tatsu@ab.jp.nec.com>
[bfields@redhat.com,nfbrown@novell.com: document instead of adding barriers]
Signed-off-by: J. Bruce Fields <bfields@redhat.com>