x86/mm: More discriptive names for page de/validation functions

x86/mm: More discriptive names for page de/validation functions

The functions alloc_page_type(), alloc_lN_table(), free_page_type()
and free_lN_table() are confusingly named: nothing is being allocated
or freed.  Rather, the page being passed in is being either validated
or devalidated for use as the specific type; in the specific case of
pagetables, these may be promoted or demoted (i.e., grab appropriate
references for PTEs).

Rename alloc_page_type() and free_page_type() to validate_page() and
devalidate_page().  Also rename alloc_segdesc_page() to
validate_segdesc_page(), since this is what it's doing.

Rename alloc_lN_table() and free_lN_table() to promote_lN_table() and
demote_lN_table(), respectively.

After this change:
- get / put type consistenly refer to increasing or decreasing the count
- validate / devalidate consistently refers to actions done when a
type count goes 0 -> 1 or 1 -> 0
- promote / demote consistenly refers to acquiring or freeing
resources (in the form of type refs and general references) in order
to allow a page to be used as a pagetable.

No functional change.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>