util/disk_cache: use fallocate() for index files on linux

ftruncate() allocates disk space lazily. If the disk is full and it is unable to allocate disk space when accesed via mmap(), it will crash with a SIGBUS.

Switch to posix_fallocate(), which ensures disk space is allocated otherwise it fails if there isn't enough disk space. The disk cache won't be enabled in this case.

For normal cases, a small increase in disk usage as the 1.3MB index file will be fully allocated when initialized now.

fallback to ftruncate() if posix_fallocate() isn't found.