memory: fix XENMEM_exchange error handling

memory: fix XENMEM_exchange error handling

assign_pages() can fail due to the domain getting killed in parallel,
which should not result in a hypervisor crash.

Reported-by: Julien Grall <julien.grall@citrix.com>
Also delete a redundant put_gfn() - all relevant paths leading to the
"fail" label already do this (and there are also paths where it was
plain wrong). All of the put_gfn()-s got introduced by 51032ca058
("Modify naming of queries into the p2m"), including the otherwise
unneeded initializer for k (with even a kind of misleading comment -
the compiler warning could actually have served as a hint that the use
is wrong).

This is CVE-2015-8339 + CVE-2015-8340 / XSA-159.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>