Revert "arm64/mm: Permit lazy_mmu_mode to be nested"

Commit 491344301b25 ("arm64/mm: Permit lazy_mmu_mode to be nested") made
the arm64 implementation of lazy_mmu_mode tolerant to nesting. But
subsequent commits have fixed the core code to ensure that lazy_mmu_mode
never gets nested (as originally intended). Therefore we can revert this
commit and reinstate the VM_WARN() if nesting is detected in future.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>