checkpatch: ignore __pure $Attribute

Just like "__cold", ignore the __pure gcc attribute macro so pointer
warnings aren't generated for uses like "int * __pure fn(...)"

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>