kernel/fork.c: check error and return early

Thus reducing one indentation level while maintaining the same rationale.

Link: http://lkml.kernel.org/r/20171117002929.5155-1-marcos.souza.org@gmail.com


Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>