perf: remember a filesystem without reflink and say so once

[CMGS]

ReflinkCopy tried FICLONE on every disk copy and fell back to SparseCopy silently, so on an ext4 root every clone paid a create, ioctl and unlink for nothing and the operator never learned that clones copy their disks in full (audit item cocoon-host D1).

• The destination directory's filesystem id is cached the first time FICLONE answers EOPNOTSUPP or ENOTTY; later copies on that filesystem skip the attempt. Only those two errnos mark a filesystem: a cross-device or per-file failure still falls back without poisoning the cache.
• One warning per filesystem names the directory and the errno.
• ReflinkCopy takes the caller's context for that log; both in-repo callers already had one.
• Linux test: after one copy in a temp dir, the cache agrees with a fresh FICLONE on the same directory (holds on ext4, tmpfs, btrfs and xfs alike). Run on the .79 Linux host (ext4): PASS.

Consumers: cocoon-macos calls utils.ReflinkCopy at two sites (cmd/vm/datadisk.go, cmd/vm/utils.go); they gain the context argument when cocoon-macos next bumps its cocoon pin.

Hot path: one statfs per disk copy on the clone path (a few µs) and, on a reflink-less filesystem, three syscalls fewer per copy.