KVM: Clarify meaning of hva_to_pfn()'s 'atomic' parameter

The current description can be read as "atomic -> allowed to sleep,"
when in fact the intended statement is "atomic -> NOT allowed to sleep."
Make that clearer in the docstring.

Signed-off-by: Anish Moorthy <amoorthy@google.com>
Link: https://lore.kernel.org/r/20240215235405.368539-2-amoorthy@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>