flat: use IS_ERR_VALUE() helper macro

There is a common macro now for testing mixed pointer/errno values, so use
that rather than handling the casts ourself.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: David McCullough <david_mccullough@securecomputing.com>
Acked-by: Greg Ungerer <gerg@uclinux.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>