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.

Reviewed by: andrew, kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51353