SUNRPC: Fix a compile error when #undef CONFIG_PROC_FS

The definition of rpc_count_iostats_metrics() is borked.

Reported by: Jim Davis <jim.epost@gmail.com>
Fixes: d67ae825 ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Cc: Tom Haynes <thomas.haynes@primarydata.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>