test-genzeros: avoid raw write(2)
This test helper feeds 256kB of data at once to a single invocation
of the write(2) system call, which may be too much for some
platforms.
Call our xwrite() wrapper that knows to honor MAX_IO_SIZE limit and
cope with short writes due to EINTR instead, and die a bit more
loudly by calling die_errno() when xwrite() indicates an error.
Reported-by: Randall S. Becker <rsbecker@nexbridge.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>