dm vdo memory-alloc: rename vdo_do_allocation to __vdo_do_allocation

__vdo_do_allocation shouldn't be used outside of memory-alloc.h, so
add hidden prefix.

Also, tabify the vdo_allocate_extended macro.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>