fs:ext4:write: Initialize cache aligned filename buffer

The filename buffer is allocated dynamically. It must be cache aligned.
Moreover, it is necessary to erase its content before we use it for
file name operations.

This prevents from corruption of written file names.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>