Simplify boot pages management in UMA.

It is simply a contigous virtual memory pointer and number of pages.
There is no need to build a linked list here. Just increment pointer
and decrement counter. The only functional difference to old allocator
is that before we gave pages from topmost and down to lowest, and now
we give them in normal ascending order.

While here remove padalign from a mutex that is unused at runtime.

Reviewed by: alc