ext4/051: correct kernel commit

ext4/051: correct kernel commit

ext4/051 fails on my test machine, but I can't find commit in
e2fsprogs according to commit log that said it is a e2fsprog
regression test. Actually, it is a kernel regression test.  So
remind user it is a kernel regression test and use correct fix
kernel commit[1].

[1]https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=b2bbb92f

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>