KVM: MMU: handle 'map_writable' in set_spte() function

Move the operation of 'writable' to set_spte() to clean up code

[avi: remove unneeded booleanification]

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>