ext2/3: use prandom_u32() instead of get_random_bytes()

Many of the uses of get_random_bytes() do not actually need
cryptographically secure random numbers.  Replace those uses with a
call to prandom_u32(), which is faster and which doesn't consume
entropy from the /dev/random driver.

The commit dd1f723b has made that for
ext4, and i did the same for ext2/3.

Signed-off-by: Zhang Zhen <zhenzhang.zhang@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>