arch/arm64/mmu: fix page table reference counting part 3

Commit f7e11649 ("arch/arm64/mmu: fix page table reference
counting") missed another case where the freeing of a whole table
"branch" didn't take into account the fact that some sub-tables might
be shared and therefore must be cleared only if the reference count is
down to 1.

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