block: two block layer core fixes and one null_blk fix
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2203606
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2212674
Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2213370
Signed-off-by: Ming Lei ming.lei@redhat.com