x86/p2m: paging_write_p2m_entry() is a private function

x86/p2m: paging_write_p2m_entry() is a private function

As it gets installed by p2m_pt_init(), it doesn't need to live in
paging.c. The function working in terms of l1_pgentry_t even further
indicates its non-paging-generic nature. Move it and drop its
paging_ prefix, not adding any new one now that it's static.

This then also makes more obvious that in the EPT case we wouldn't
risk mistakenly calling through the NULL hook pointer.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>