NFSD: Fix sparse warning in nfs4proc.c

linux/fs/nfsd/nfs4proc.c:1542:24: warning: incorrect type in assignment (different base types)
linux/fs/nfsd/nfs4proc.c:1542:24:    expected restricted __be32 [assigned] [usertype] status
linux/fs/nfsd/nfs4proc.c:1542:24:    got int

Clean-up: The dup_copy_fields() function returns only zero, so make
it return void for now, and get rid of the return code check.

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