NFSv4: Make _nfs42_proc_copy_notify() static

Fix sparse warning:

fs/nfs/nfs42proc.c:527:5: warning:
 symbol '_nfs42_proc_copy_notify' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>