gfs2: Silence "suspicious RCU usage in gfs2_permission" warning

Commit 0abd1557 added rcu_dereference() for dereferencing ip->i_gl
in gfs2_permission.  This now causes lockdep to complain when
gfs2_permission is called in non-RCU context:

    WARNING: suspicious RCU usage in gfs2_permission

Switch to rcu_dereference_check() and check for the MAY_NOT_BLOCK flag
to shut up lockdep when we know that dereferencing ip->i_gl is safe.

Fixes: 0abd1557 ("gfs2: fix an oops in gfs2_permission")
Reported-by: syzbot+3e5130844b0c0e2b4948@syzkaller.appspotmail.com
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>

fs/gfs2/inode.c

@@ -1867,6 +1867,7 @@ static const char *gfs2_get_link(struct dentry *dentry,
 int gfs2_permission(struct mnt_idmap *idmap, struct inode *inode,
 		    int mask)
 {
+	int may_not_block = mask & MAY_NOT_BLOCK;
 	struct gfs2_inode *ip;
 	struct gfs2_holder i_gh;
 	struct gfs2_glock *gl;
@@ -1874,14 +1875,14 @@ int gfs2_permission(struct mnt_idmap *idmap, struct inode *inode,
 
 	gfs2_holder_mark_uninitialized(&i_gh);
 	ip = GFS2_I(inode);
-	gl = rcu_dereference(ip->i_gl);
+	gl = rcu_dereference_check(ip->i_gl, !may_not_block);
 	if (unlikely(!gl)) {
 		/* inode is getting torn down, must be RCU mode */
-		WARN_ON_ONCE(!(mask & MAY_NOT_BLOCK));
+		WARN_ON_ONCE(!may_not_block);
 		return -ECHILD;
         }
 	if (gfs2_glock_is_locked_by_me(gl) == NULL) {
-		if (mask & MAY_NOT_BLOCK)
+		if (may_not_block)
 			return -ECHILD;
 		error = gfs2_glock_nq_init(gl, LM_ST_SHARED, LM_FLAG_ANY, &i_gh);
 		if (error)