f2fs: avoid variable length array

Instead of using variable length array, this patch let preallocate memory for
them.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

diff --cc fs/f2fs/debug.c
Simple merge

diff --cc fs/f2fs/segment.c
Simple merge

diff --cc fs/f2fs/segment.h
Simple merge