syscall: golf + more idiomatic buffer initialization

syscall: golf + more idiomatic buffer initialization

While `vec' is useful for user-supplied buffers to avoid excess
memory traffic, but provides no benefit when we need to allocate
our own buffers as we do in nodatacow_fh, since Perl can't elide
memset(ptr, 0, len).  So just use the idiomatic `"\0" x $LEN' here.