Revert "ext4: Speedup WB_SYNC_ALL pass called from sync(2)"

This reverts commit 5d5aed87018ccb65179e8fb36f07d4ec59f26c42.
Revert "ext4: use prandom_u32() instead of get_random_bytes()"

This reverts commit ec8b40ebcbbf6eb43aee42b39bbecc1973ab3b78.