Use pmap_pte_exists() instead of pmap_pte() when the caller expects a mapping to exist at a particular level. The caller benefits in two ways from using pmap_pte_exists(). First, because the level is specified to pmap_pte_exists() as a constant, rather than returned, the compiler can specialize the implementation of pmap_pte_exists() to the caller's exact needs, i.e., generate fewer instructions. Consequently, within a GENERIC-NODEBUG kernel, 704 bytes worth of instructions are eliminated from the inner loops of various pmap functions. Second, suppose that the mapping doesn't exist. Rather than requiring every caller to implement its own KASSERT()s to report missing mappings, the caller can optionally have pmap_pte_exists() provide the KASSERT().