sparc: fix the return value of module_alloc()

In case of error, function module_alloc() in other platform never
returns ERR_PTR(), and all of the user only check for NULL, so
we'd better return NULL instead of ERR_PTR().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>