f2fs: replace count*size kzalloc by kcalloc

kcalloc manages count*sizeof overflow.

Cc: Jaegeuk Kim <jaegeuk.kim@samsung.com>
Cc: linux-f2fs-devel@lists.sourceforge.net
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>