Always clear L1 PTE descriptor when removing superpage on ARM

Invalidate L1 PTE regardles of existance of the corresponding
l2_bucket. This is relevant when superpage is entered via
pmap_enter_object() and will fix crash on entering page
in place of not properly removed superpage.
Assure proper TLB invalidation on superpage promotion and demotion on ARM

Base pages within newly created superpage need to be invalidated so that
new mapping is "visible" immediately after creation.
Fix superpage promotion on ARM with respect to RO/RW and wired attributes

It was possible to create RW superpage mapping even if
the base pages were RO due to wrong setting of the prot
flag passed to pmap_map_section().
Promotion attempt should be canceled in case of attributes
mismatch between any two base pages. Since we still use
pv_flags to maintain permission to write (PVF_WRITE) and
wired status (PVF_WIRED) for a page, it is also necessary
to take those variables into account.
Avoid redundant superpage promotion attempts on ARM

Because pmap_enter_locked() is called from few different functions
some redundancy in superpage promotion attempts can be observed.
Hence, avoid promotion in pmap_enter_object() (if the object can
be mapped by superpage it will be handled by pmap_enter_object()
itself) and also do not waste time in pmap_enter_quick().
From now on the promotion will be performed only in pmap_enter().
Remove spurious assertion from pmap_extract_locked()

The condition under assertion is no longer valid since
superpages support is operating on section mappings.
Handle pmap_enter() on already promoted mappings for ARMv6/v7

Try to demote the superpage if trying to pmap_enter() on
one. Panic only when the particular superpage should
no longer exist for that pmap and address.