qcow2: Allow 'cache-clean-interval' in Linux only

The cache-clean-interval option of qcow2 only works on Linux. However
we allow setting it in other systems regardless of whether it works or
not.

In those systems this option is not simply a no-op: it actually
invalidates perfectly valid cache tables for no good reason without
freeing their memory.

This patch forbids using that option in non-Linux systems.
Signed-off-by: NAlberto Garcia <berto@igalia.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/qcow2.c

@@ -668,6 +668,14 @@ static int qcow2_update_options_prepare(BlockDriverState *bs,
     r->cache_clean_interval =
         qemu_opt_get_number(opts, QCOW2_OPT_CACHE_CLEAN_INTERVAL,
                             s->cache_clean_interval);
+#ifndef CONFIG_LINUX
+    if (r->cache_clean_interval != 0) {
+        error_setg(errp, QCOW2_OPT_CACHE_CLEAN_INTERVAL
+                   " not supported on this host");
+        ret = -EINVAL;
+        goto fail;
+    }
+#endif
     if (r->cache_clean_interval > UINT_MAX) {
         error_setg(errp, "Cache clean interval too big");
         ret = -EINVAL;