x86/mem-sharing: Don't leave the altp2m lock held when nominating a page

x86/mem-sharing: Don't leave the altp2m lock held when nominating a page

get_gfn_type_access() internally takes the p2m lock, and nothing ever unlocks
it.  Switch to using the unlocked accessor instead.

This wasn't included in XSA-277 because neither mem-sharing nor altp2m are
supported.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Tamas K Lengyel <tamas@tklengyel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
master commit: d6e02850d3b45c9658457214a749cc48097bdef4
master date: 2018-11-22 17:58:46 +0000