tmem: Fix uses of unmatched __map_domain_page()

tmem: Fix uses of unmatched __map_domain_page()

__map_domain_page() *must* be matched with an unmap_domain_page().  These five
static inline functions each map a page (or two), then throw away the context
needed to unmap it.

Each of the changes are limited to their respective functions.  In two cases,
this involved replacing a large amount of pointer arithmetic with memcpy()
(all callers were relying on memcpy() semantics of positive/negative returns
rather than specifically -1/+1). A third case had its pointer arithmetic
entirely replaced with memcpy().

In addition, remove redundant casts of void pointers and assertions.

This fixes Coverity IDs 1135373 1135374 1135375 1135376 1135377 1135378
11353739 which were retroactively identified following modelling improvements.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Tested-by: Bob Liu <bob.liu@oracle.com>