UBIFS: fix compilation warning

Fix the following compilation warning:

fs/ubifs/dir.c: In function 'ubifs_rename':
fs/ubifs/dir.c:972:15: warning: 'saved_nlink' may be used uninitialized
in this function

Use the 'uninitialized_var()' macro to get rid of this false-positive.

Artem: massaged the patch a bit.

Signed-off-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>