sunrpc: fix missing kernel-doc

Fix missing sunrpc kernel-doc:

Warning(linux-2.6.25-git7//net/sunrpc/xprt.c:451): No description found for parameter 'action'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>