sunrpc: raise kernel RPC channel buffer size

Its possible that using AUTH_SYS and mountd manage-gids option a
user may hit the 8k RPC channel buffer limit. This have been observed
on field, causing unanswered RPCs on clients after mountd fails to
write on channel :

rpc.mountd[11231]: auth_unix_gid: error writing reply

Userland nfs-utils uses a buffer size of 32k (RPC_CHAN_BUF_SIZE), so
lets match those two.
Signed-off-by: NRoberto Bergantinos Corpas <rbergant@redhat.com>
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

net/sunrpc/cache.c

@@ -909,7 +909,7 @@ static ssize_t cache_do_downcall(char *kaddr, const char __user *buf,
 static ssize_t cache_slow_downcall(const char __user *buf,
 				   size_t count, struct cache_detail *cd)
 {
-	static char write_buf[8192]; /* protected by queue_io_mutex */
+	static char write_buf[32768]; /* protected by queue_io_mutex */
 	ssize_t ret = -EINVAL;
 
 	if (count >= sizeof(write_buf))