lib/vmalloc: add locking and a check for initialization

Make sure init_alloc_vpage is never called when vmalloc is in use.

Get both init_alloc_vpage and setup_vm to use the lock.

For setup_vm we only check at the end because at least on some
architectures setup_mmu can call init_alloc_vpage, which would cause
a deadlock.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200622162141.279716-9-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>