x86: change x86_64 smp_call_function_mask to look alike i386

the two versions (the inner version, and the outer version, that takes
the locks) of smp_call_function_mask are made into one. With the changes,
i386 and x86_64 versions look exactly the same.

Signed-off-by: Glauber Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>