Revert "arm64/mm: Drop redundant BUG_ON(!pgtable_alloc)"

This reverts commit 9ed2b461.

Nathan reports early boot failures bisected to this change which look
related to the kPTI nG repainting. In any case, consolidating the
BUG_ON()s to a single location needs more thought, so revert the change
until this is figured out properly.

Link: https://lore.kernel.org/r/Y3pS5fdZ3MdLZ00t@dev-arch.thelio-3990X


Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>