KVM: SVM: Rename get_max_npt_level() to get_npt_level()

It returns the only proper NPT level, so the "max" in the name
is not appropriate.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20211118110814.2568-9-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>