For the upgrade case in vm_fault_copy_entry(), when the entry does not

For the upgrade case in vm_fault_copy_entry(), when the entry does not
need COW and is writeable (i.e. becoming writeable due to the
mprotect(2) operation), do not create a new backing object for the
entry.  The caller of the function is vm_map_protect(), the call is
made to ensure that wired entry has all pages resident and wired in
the top level object and to enable the write.  We might need to copy
read-only page from some backing objects into the top object or remap
the page with the write allowed.

This fixes the issue with mishandling of the swap accounting when
read-only wired mapping is upgraded to write-enabled after fork.  The
previous code path did not accounted the new object, but it creation
is redundand anyway and the change provides an optimization for the
non-common situation.

Reported by:	markj
Suggested and reviewed by:	alc (previous version)
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week