gfs2: read-only mounts should grab the sd_freeze_gl glock

Before this patch, only read-write mounts would grab the freeze
glock in read-only mode, as part of gfs2_make_fs_rw. So the freeze
glock was never initialized. That meant requests to freeze, which
request the glock in EX, were granted without any state transition.
That meant you could mount a gfs2 file system, which is currently
frozen on a different cluster node, in read-only mode.

This patch makes read-only mounts lock the freeze glock in SH mode,
which will block for file systems that are frozen on another node.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/ops_fstype.c

@@ -1136,7 +1136,17 @@ static int gfs2_fill_super(struct super_block *sb, struct fs_context *fc)
 		goto fail_per_node;
 	}
 
-	if (!sb_rdonly(sb)) {
+	if (sb_rdonly(sb)) {
+		struct gfs2_holder freeze_gh;
+
+		error = gfs2_glock_nq_init(sdp->sd_freeze_gl, LM_ST_SHARED,
+					   GL_EXACT, &freeze_gh);
+		if (error) {
+			fs_err(sdp, "can't make FS RO: %d\n", error);
+			goto fail_per_node;
+		}
+		gfs2_glock_dq_uninit(&freeze_gh);
+	} else {
 		error = gfs2_make_fs_rw(sdp);
 		if (error) {
 			fs_err(sdp, "can't make FS RW: %d\n", error);