x86/shadow: polish shadow_write_entries()

x86/shadow: polish shadow_write_entries()

First of all, avoid the initial dummy write: Try to write the actual
new value instead, and start the loop from 1 if this was successful.
Further, drop safe_write_entry() and use write_atomic() instead. This
eliminates the need for the BUILD_BUG_ON() there at the same time.

Then
- use const and unsigned,
- drop a redundant NULL check,
- don't open-code PAGE_OFFSET() and IS_ALIGNED(),
- adjust comment style.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>