pmap_enter_{l2,pde}: correct the handling of an error case

When pmap_enter_object()'s call to pmap_enter_{l2,pde}() fails to create
a managed mapping within the kernel address space due to the inability
to allocate a PV entry, it needs to remove the kernel page table page
from the pmap's trie of idle page table pages. Previously, it did not.

(cherry picked from commit 5a846c48f209d04dad36aa35a9968f557c4516f7)