fs/xfs: Ensuring failing to mount sets a grub_errno

It was previously possible for grub_xfs_mount() to return NULL without
setting grub_errno if the XFS version was invalid. This resulted in it
being possible for grub_dl_unref() to be called twice allowing the XFS
module to be unloaded while there were still references to it.

Fixing this problem in general by ensuring a grub_errno is set if the
fail label is reached.

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/xfs.c

@@ -327,6 +327,8 @@ static int grub_xfs_sb_valid(struct grub_xfs_data *data)
 	}
       return 1;
     }
+
+  grub_error (GRUB_ERR_BAD_FS, "unsupported XFS filesystem version");
   return 0;
 }
 
@@ -1068,7 +1070,7 @@ grub_xfs_mount (grub_disk_t disk)
   return data;
  fail:
 
-  if (grub_errno == GRUB_ERR_OUT_OF_RANGE)
+  if (grub_errno == GRUB_ERR_OUT_OF_RANGE || grub_errno == GRUB_ERR_NONE)
     grub_error (GRUB_ERR_BAD_FS, "not an XFS filesystem");
 
   grub_free (data);