x86/mm: Refactor modify_xen_mappings to have one exit path

x86/mm: Refactor modify_xen_mappings to have one exit path

We will soon need to perform clean-ups before returning.

No functional change.

This is part of XSA-345.

Reported-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: b733f8a8b8db83f2d438cab3adb38b387cecfce0
master date: 2020-10-20 14:19:55 +0200