fs/f2fs: Set a grub_errno if mount fails

It was previously possible for grub_errno to not be set when
grub_f2fs_mount() failed if nat_bitmap_ptr() returned NULL.

This issue is solved by ensuring a grub_errno is set in the fail case.

Reported-by: B Horn <b@horn.uk>
Signed-off-by: B Horn <b@horn.uk>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/fs/f2fs.c

@@ -872,6 +872,9 @@ grub_f2fs_mount (grub_disk_t disk)
   return data;
 
  fail:
+  if (grub_errno == GRUB_ERR_NONE)
+    grub_error (GRUB_ERR_BAD_FS, "not a F2FS filesystem");
+
   grub_free (data);
 
   return NULL;