KVM: x86/mmu: remove unnecessary "bool shared" argument from functions

Neither tdp_mmu_next_root nor kvm_tdp_mmu_put_root need to know
if the lock is taken for read or write.  Either way, protection
is achieved via RCU and tdp_mmu_pages_lock.  Remove the argument
and just assert that the lock is taken.

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20231125083400.1399197-2-pbonzini@redhat.com
Signed-off-by: Sean Christopherson <seanjc@google.com>