x86/mem-sharing: mov {get,put}_two_gfns()

x86/mem-sharing: mov {get,put}_two_gfns()

There's no reason for every CU including p2m.h to have these two
functions compiled, when they're both mem-sharing specific right now and
for the foreseeable future.

Largely just code movement, with some style tweaks, the inline-s
dropped, and "put" being made consistent with "get" as to their NULL
checking of the passed in pointer to struct two_gfns.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>