UBIFS: add missing ui pointer in debugging code

If UBIFS_DEBUG is defined an additional assertion of the ui_lock
spinlock in do_writepage cannot compile because the ui pointer has not
been previously declared.

Fix this by declaring and initializing the ui pointer in case
UBIFS_DEBUG is defined.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>