arm64: fix a potential KVA leak in pmap_demote_l1()

In the unlikely event that the 1 GB page mapping being demoted is used
to access the L1 page table page containing the 1 GB page mapping and
the vm_page_alloc() to allocate a new L2 page table page fails, we
would leak a page of kernel virtual address space. Fix this leak.

MFC after: 1 week