Correctly implement PMAP_ENTER_NOREPLACE in pmap_enter_{l2,pde}() on kernel
mappings.

Reduce code duplication by defining a function, pmap_abort_ptp(), for
handling a common error case.

Simplify error handling in pmap_enter_quick_locked().

Reviewed by: kib
Tested by: pho
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D22890