cifs: properly invalidate cached root handle when closing it

Cached root file was not being completely invalidated sometimes.

Reproducing:
- With a DFS share with 2 targets, one disabled and one enabled
- start some I/O on the mount
  # while true; do ls /mnt/dfs; done
- at the same time, disable the enabled target and enable the disabled
  one
- wait for DFS cache to expire
- on reconnect, the previous cached root handle should be invalid, but
  open_cached_dir_by_dentry() will still try to use it, but throws a
  use-after-free warning (kref_get())

Make smb2_close_cached_fid() invalidate all fields every time, but only
send an SMB2_close() when the entry is still valid.
Signed-off-by: Enzo Matsumiya <ematsumiya@suse.de>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2ops.c

@@ -689,6 +689,13 @@ smb2_close_cached_fid(struct kref *ref)
 		cifs_dbg(FYI, "clear cached root file handle\n");
 		SMB2_close(0, cfid->tcon, cfid->fid->persistent_fid,
 			   cfid->fid->volatile_fid);
+	}
+
+	/*
+	 * We only check validity above to send SMB2_close,
+	 * but we still need to invalidate these entries
+	 * when this function is called
+	 */
 	cfid->is_valid = false;
 	cfid->file_all_info_is_valid = false;
 	cfid->has_lease = false;
@@ -696,7 +703,6 @@ smb2_close_cached_fid(struct kref *ref)
 		dput(cfid->dentry);
 		cfid->dentry = NULL;
 	}
-	}
 }
 
 void close_cached_dir(struct cached_fid *cfid)