arch/arm64/mmu: fix page table reference counting part 2

Commit f7e11649 ("arch/arm64/mmu: fix page table reference
counting") missed a case where the freeing of a table wasn't propagated
properly to all domains. To fix this, the page freeing logic was removed
from set_mapping() and a del_mapping() was created instead, to be usedby
both by remove_map() and globalize_table().

A test covering this case should definitely be created but that'll come
later. Proper operation was verified through manual debug log
inspection for now.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>