cifs: check pointer before freeing

clang static analysis reports this problem

dfs_cache.c:591:2: warning: Argument to kfree() is a constant address
  (18446744073709551614), which is not memory allocated by malloc()
        kfree(vi);
        ^~~~~~~~~

In dfs_cache_del_vol() the volume info pointer 'vi' being freed
is the return of a call to find_vol().  The large constant address
is find_vol() returning an error.

Add an error check to dfs_cache_del_vol() similar to the one done
in dfs_cache_update_vol().

Fixes: 54be1f6c1c ("cifs: Add DFS cache routines")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
CC: <stable@vger.kernel.org> # v5.0+
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/dfs_cache.c

@@ -1260,7 +1260,8 @@ void dfs_cache_del_vol(const char *fullpath)
 	vi = find_vol(fullpath);
 	spin_unlock(&vol_list_lock);
 
-	kref_put(&vi->refcnt, vol_release);
+	if (!IS_ERR(vi))
+		kref_put(&vi->refcnt, vol_release);
 }
 
 /**