sunrpc: fix stats.h for CONFIG_PROC_FS not enabled

Fix build errors in linux/sunrpc/stats.h when CONFIG_PROC_FS
is not enabled:
- add parameter names to inline functions
- fix placement of '(' in rpc_proc_unregister()

Fixes these errors:
include/linux/sunrpc/stats.h:72:63: error: parameter name omitted
include/linux/sunrpc/stats.h:73:46: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'net'

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>