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>

fs/gfs2/ops_fstype.c

@@ -687,6 +687,7 @@ static int init_statfs(struct gfs2_sbd *sdp)
 	}
 
 	iput(pn);
+	pn = NULL;
 	ip = GFS2_I(sdp->sd_sc_inode);
 	error = gfs2_glock_nq_init(ip->i_gl, LM_ST_EXCLUSIVE, 0,
 				   &sdp->sd_sc_gh);