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

This reverts commit 9ed2b4616d4e846ece2a04cb5007ce1d1bd9e3f3.

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/[e-mail]-3990X
Reported-by: Nathan Chancellor <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

arch/arm64/mm/mmu.c

···
 
 		if (flags & NO_EXEC_MAPPINGS)
 			pmdval |= PMD_TABLE_PXN;
+		BUG_ON(!pgtable_alloc);
 		pte_phys = pgtable_alloc(PAGE_SHIFT);
 		__pmd_populate(pmdp, pte_phys, pmdval);
 		pmd = READ_ONCE(*pmdp);
···
 
 		if (flags & NO_EXEC_MAPPINGS)
 			pudval |= PUD_TABLE_PXN;
+		BUG_ON(!pgtable_alloc);
 		pmd_phys = pgtable_alloc(PMD_SHIFT);
 		__pud_populate(pudp, pmd_phys, pudval);
 		pud = READ_ONCE(*pudp);
···
 
 		if (flags & NO_EXEC_MAPPINGS)
 			p4dval |= P4D_TABLE_PXN;
+		BUG_ON(!pgtable_alloc);
 		pud_phys = pgtable_alloc(PUD_SHIFT);
 		__p4d_populate(p4dp, pud_phys, p4dval);
 		p4d = READ_ONCE(*p4dp);
···
 	phys &= PAGE_MASK;
 	addr = virt & PAGE_MASK;
 	end = PAGE_ALIGN(virt + size);
-	BUG_ON(!pgtable_alloc);
 
 	do {
 		next = pgd_addr_end(addr, end);