KVM: arm64: Don't hold 'vm_table_lock' across guest page reclaim

Now that the teardown of a VM cannot be finalised as long as a reference
is held on the VM, rework __pkvm_reclaim_dying_guest_page() to hold a
reference to the dying VM rather than take the global 'vm_table_lock'
during the reclaim operation.

Signed-off-by: Will Deacon <will@kernel.org>