Don't assume the current root to be valid, just check it and remove
the WARN().

Also move the code to check if the root is valid into FNAME(invlpg)
to simplify the code.
Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Link: https://lore.kernel.org/r/20230216235321.735214-2-jiangshanlai@gmail.comSigned-off-by: Sean Christopherson <seanjc@google.com>