ext4: correctly zero filename

Since ALLOC_CACHE_ALIGN_BUFFER declares a char* for filename
sizeof(filename) is not the size of the buffer. Use the already
known length instead.

cc: Uma Shankar <uma.shankar@samsung.com>
cc: Manjunatha C Achar <a.manjunatha@samsung.com>
cc: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Jeroen Hofstee <jeroen@myspectrum.nl>
Acked-by: Marek Vasut <marex@denx.de>