arch/arm64/mmu: fix table discarding code

First, we have commit 7d27bd0b ("arch: arm64: Disable infinite
recursion warning for `discard_table`") that blindly shut up a compiler
warning that did actually highlighted a real bug. Revert that and fix
the bug properly. And yes, mea culpa for having been the first to
approve that commit, or even creating the bug in the first place.

Then let's add proper table usage cound handling for discard_table() to
work properly and avoid leaking table pages.

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