fs/romfs: correct error-handling code

romfs_iget returns an ERR_PTR value in an error case instead of NULL.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/)

// <smpl>
@match exists@
expression x, E;
statement S1, S2;
@@

x = romfs_iget(...)
... when != x = E
(
* if (x == NULL || ...) S1 else S2
|
* if (x == NULL && ...) S1 else S2
)
// </smpl>

Signed-off-by: Julia Lawall <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/romfs/super.c

···
 	pos = (ROMFH_SIZE + len + 1 + ROMFH_PAD) & ROMFH_MASK;
 
 	root = romfs_iget(sb, pos);
-	if (!root)
+	if (IS_ERR(root))
 		goto error;
 
 	sb->s_root = d_alloc_root(root);