ubifs: avoid assert failed in ubifs.c

This patch solves assert failed displayed in the console during a boot.
The root cause is that the ubifs_inode is not already allocated when
ubifs_printdir and ubifs_finddir functions are called.

Trace showing the issue:
feed 'boot.scr.uimg', ino 94, new f_pos 0x17b40ece
dent->ch.sqnum '7132', creat_sqnum 3886945402880
UBIFS assert failed in ubifs_finddir at 436
INODE ALLOCATION: creat_sqnum '7129'
Found U-Boot script /boot.scr.uimg

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>