SUNRPC: Address RCU warning in net/sunrpc/svc.c

$ make C=1 W=1 net/sunrpc/svc.o
make[1]: Entering directory 'linux/obj/manet.1015granger.net'
  GEN     Makefile
  CALL    linux/server-development/scripts/checksyscalls.sh
  DESCEND objtool
  INSTALL libsubcmd_headers
  DESCEND bpf/resolve_btfids
  INSTALL libsubcmd_headers
  CC [M]  net/sunrpc/svc.o
  CHECK   linux/server-development/net/sunrpc/svc.c
linux/server-development/net/sunrpc/svc.c:1225:9: warning: incorrect type in argument 1 (different address spaces)
linux/server-development/net/sunrpc/svc.c:1225:9:    expected struct spinlock [usertype] *lock
linux/server-development/net/sunrpc/svc.c:1225:9:    got struct spinlock [noderef] __rcu *
linux/server-development/net/sunrpc/svc.c:1227:40: warning: incorrect type in argument 1 (different address spaces)
linux/server-development/net/sunrpc/svc.c:1227:40:    expected struct spinlock [usertype] *lock
linux/server-development/net/sunrpc/svc.c:1227:40:    got struct spinlock [noderef] __rcu *
make[1]: Leaving directory 'linux/obj/manet.1015granger.net'

Warning introduced by commit 913292c9 ("sched.h: Annotate
sighand_struct with __rcu").

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>