xen/p2m: Don't call get_balloon_scratch_page() twice, keep interrupts disabled for multicalls

m2p_remove_override() calls get_balloon_scratch_page() in
MULTI_update_va_mapping() even though it already has pointer to this page from
the earlier call (in scratch_page). This second call doesn't have a matching
put_balloon_scratch_page() thus not restoring preempt count back. (Also, there
is no put_balloon_scratch_page() in the error path.)

In addition, the second multicall uses __xen_mc_entry() which does not disable
interrupts. Rearrange xen_mc_* calls to keep interrupts off while performing
multicalls.

This commit fixes a regression introduced by:

commit ee072640
Author: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Date:   Tue Jul 23 17:23:54 2013 +0000

    xen/m2p: use GNTTABOP_unmap_and_replace to reinstate the original mapping

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>