coredump: use realloc() loop instead of malloc(768M)

coredump: use realloc() loop instead of malloc(768M)

I typically run VMs with 1024MiB allocated; systemd is unable to write
coredumps in this scenario at all because the default kernel
configuration will only overcommit 50% of available RAM.

Avoid this failure by using a realloc() loop.

See: http://lists.freedesktop.org/archives/systemd-devel/2013-April/010709.html