gfs2: Fix error handling in init_statfs

On an error path, init_statfs calls iput(pn) after pn has already been put.
Fix that by setting pn to NULL after the initial iput.

Fixes: 97fd734b ("gfs2: lookup local statfs inodes prior to journal recovery")
Cc: stable@vger.kernel.org # v5.10+
Reported-by: Jing Xiangfeng <jingxiangfeng@huawei.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>