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.

Fixes: ce40733c ("ext4: Check for return value from sb_set_blocksize")
Fixes: ac27a0ec

 ("ext4: initial copy of files from ext3")
Link: https://lore.kernel.org/r/20210521075533.95732-1-amakhalov@vmware.com
Signed-off-by: Alexey Makhalov <amakhalov@vmware.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org