gfs2: Refcounting fix in gfs2_thaw_super

It turns out that the .freeze_super and .thaw_super operations require
the filesystem to manage the superblock refcount itself.  We are using
the freeze_super() and thaw_super() helpers to mostly take care of that
for us, but this means that the superblock may no longer be around by
when thaw_super() returns, and gfs2_thaw_super() will then access freed
memory.  Take an extra superblock reference in gfs2_thaw_super() to fix
that.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>