CVE-2021-47119: ext4: fix memory leak in ext4_fill_super

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

ext4: fix memory leak in ext4fillsuper

Buffer head references must be released before calling killbdev(); otherwise the buffer head (and its page referenced by bdata) will not be freed by killbdev, and subsequently that bh will be leaked.

If blocksizes differ, sbsetblocksize() will kill current buffers and page cache by using killbdev(). And then super block will be reread again but using correct blocksize this time. sbsetblocksize() 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 <ext4onlvm>.mount, and systemctl stop <ext4onlvm>.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.