btrfs-util: unfuck tmpfiles' subvol creation

tmpfiles now passes an O_PATH fd to btrfs_subvol_make_fd() under the
assumption it will accept it like mkdirat() does. So far this assumption
was wrong, let's correct that.

Without that tmpfiles' on btrfs file systems failed systematically...

src/basic/btrfs-util.c

@@ -118,6 +118,7 @@ int btrfs_is_subvol(const char *path) {
 
 int btrfs_subvol_make_fd(int fd, const char *subvolume) {
         struct btrfs_ioctl_vol_args args = {};
+        _cleanup_close_ int real_fd = -1;
         int r;
 
         assert(subvolume);
@@ -126,6 +127,20 @@ int btrfs_subvol_make_fd(int fd, const char *subvolume) {
         if (r < 0)
                 return r;
 
+        r = fcntl(fd, F_GETFL);
+        if (r < 0)
+                return -errno;
+        if (FLAGS_SET(r, O_PATH)) {
+                /* An O_PATH fd was specified, let's convert here to a proper one, as btrfs ioctl's can't deal with
+                 * O_PATH. */
+
+                real_fd = fd_reopen(fd, O_RDONLY|O_CLOEXEC|O_DIRECTORY);
+                if (real_fd < 0)
+                        return real_fd;
+
+                fd = real_fd;
+        }
+
         strncpy(args.name, subvolume, sizeof(args.name)-1);
 
         if (ioctl(fd, BTRFS_IOC_SUBVOL_CREATE, &args) < 0)