include/linux/cred.h: fix build

mips allmodconfig:

include/linux/cred.h: In function `creds_are_invalid':
include/linux/cred.h:187: error: `PAGE_SIZE' undeclared (first use in this function)
include/linux/cred.h:187: error: (Each undeclared identifier is reported only once
include/linux/cred.h:187: error: for each function it appears in.)

Fixes

commit b6dff3ec5e116e3af6f537d4caedcad6b9e5082a
Author:     David Howells <dhowells@redhat.com>
AuthorDate: Fri Nov 14 10:39:16 2008 +1100
Commit:     James Morris <jmorris@namei.org>
CommitDate: Fri Nov 14 10:39:16 2008 +1100

    CRED: Separate task security context from task_struct

I think.

It's way too large to be inlined anyway.

Dunno if this needs an EXPORT_SYMBOL() yet.

Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: Serge Hallyn <serue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>