When doing pmap_enter_locked(), enable write permission only when access
type indicates attempt to write. Otherwise, leave the page read only but
mark it writable in pv_flags.
This will result in:
1. Marking page writable during pmap_enter() but only when ensured that it
will be written right away so that we will not get redundant permissions
fault on write attempt.
2. Keeping page read only when it is permitted to be written but there was
no actual write attempt. Hence, we will get permissions fault on write
access and mark page writable in pmap_fault_fixup() what will indicate
modification status.
Submitted by: Zbigniew Bodek <zbb@semihalf.com>
Sponsored by: The FreeBSD Foundation, Semihalf
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