Use IS_ERR() instead of comparing to NULL.

[akpm@linux-foundation.org: preserve the error code]
Signed-off-by: NJani Nikula <ext-jani.1.nikula@nokia.com>
Cc: Vegard Nossum <vegard.nossum@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>