ext4: fix memory leak in ext4_fill_super

In the Linux kernel, the following vulnerability has been resolved:

ext4: fix memory leak in ext4_fill_super

Buffer head references must be released before calling kill_bdev();

otherwise the buffer head (and its page referenced by b_data) will not

be freed by kill_bdev, and subsequently that bh will be leaked.

If blocksizes differ, sb_set_blocksize() will kill current buffers and

page cache by using kill_bdev(). And then super block will be reread

again but using correct blocksize this time. sb_set_blocksize() didn't

fully free superblock page and buffer head, and being busy, they were

not freed and instead leaked.

This can easily be reproduced by calling an infinite loop of:

systemctl start <ext4_on_lvm>.mount, and

systemctl stop <ext4_on_lvm>.mount

... since systemd creates a cgroup for each slice which it mounts, and

the bh leak get amplified by a dying memory cgroup that also never

gets freed, and memory consumption is much more easily noticed.