ovl: fix bogus -Wmaybe-unitialized warning

gcc gets a bit confused by the logic in ovl_setup_trap() and
can't figure out whether the local 'trap' variable in the caller
was initialized or not:

fs/overlayfs/super.c: In function 'ovl_fill_super':
fs/overlayfs/super.c:1333:4: error: 'trap' may be used uninitialized in this function [-Werror=maybe-uninitialized]
    iput(trap);
    ^~~~~~~~~~
fs/overlayfs/super.c:1312:17: note: 'trap' was declared here

Reword slightly to make it easier for the compiler to understand.

Fixes: 146d62e5 ("ovl: detect overlapping layers")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/super.c

@@ -995,8 +995,8 @@ static int ovl_setup_trap(struct super_block *sb, struct dentry *dir,
 	int err;
 
 	trap = ovl_get_trap_inode(sb, dir);
-	err = PTR_ERR(trap);
-	if (IS_ERR(trap)) {
+	err = PTR_ERR_OR_ZERO(trap);
+	if (err) {
 		if (err == -ELOOP)
 			pr_err("overlayfs: conflicting %s path\n", name);
 		return err;