btrfs_read_policy_to_enum
if (btrfs_read_policy_to_enum(read_policy, &value) == -EINVAL) {
index = btrfs_read_policy_to_enum(buf, &value);
int btrfs_read_policy_to_enum(const char *str, s64 *value);
fs_devices->read_policy = btrfs_read_policy_to_enum(btrfs_get_mod_read_policy(),