Replace __va_list with va_list

Most of the code base already uses va_list, which is specified by
iso-c. gcc/glibc provides 'typedef gnuc_va_list va_list'. and
when not using gcc/glibc we can't expect to find gnuc_va_list.

Signed-off-by: Alec Salazar <alec.j.salazar@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2588