buffer.c: call thaw_super during emergency thaw

There are 2 distinct freezing mechanisms - one operates on block
devices and another one directly on super blocks. Both end up with the
same result, but thaw of only one of these does not thaw the other.

In particular fsfreeze --freeze uses the ioctl variant going to the
super block. Since prior to this patch emergency thaw was not doing
a relevant thaw, filesystems frozen with this method remained
unaffected.

The patch is a hack which adds blind unfreezing.

In order to keep the super block write-locked the whole time the code
is shuffled around and the newly introduced __iterate_supers is
employed.

Signed-off-by: Mateusz Guzik <mguzik@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>