When pmap_enter_{l2,pde}() are called to create a kernel mapping, they are
incrementing (and decrementing) the ref_count on kernel page table pages.
They should not do this. Kernel page table pages are expected to have a
fixed ref_count. Address this problem by refactoring pmap_alloc{_l2,pde}()
and their callers. This also eliminates some duplicated code from the
callers.

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.

Handle a possible page table page leak in pmap_copy(). Suppose that we are
determining whether to copy a superpage mapping. If we abort because there
is already a mapping in the destination pmap at the current address, then
simply decrementing the page table page's ref_count is correct, because the
page table page must have a ref_count > 1. However, if we abort because we
failed to allocate a PV entry, this might be a just allocated page table
page that has a ref_count = 1, so we should call pmap_abort_ptp().

Simplify error handling in pmap_enter_quick_locked().

Reviewed by: kib, markj (an earlier)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D22763