Debian: CVE-2021-47119: linux -- security update

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 .mount, and
systemctl stop .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.