util/disk_cache: fall back to ftruncate if posix_fallocate isn't supported
After !22097 (merged) disk cache may always fail as ZFS doesn't support posix_fallocate. While GNU libc emulates posix_fallocate if not supported other libc (FreeBSD libc, musl, bionic) don't.
See also https://github.com/freebsd/freebsd-src/commit/96ed2690dfb3
Edited by Jan Beich