xen: let alloc_xenballooned_pages() fail if not enough memory free

xen: let alloc_xenballooned_pages() fail if not enough memory free

Instead of trying to allocate pages with GFP_USER in
add_ballooned_pages() check the available free memory via
si_mem_available(). GFP_USER is far less limiting memory exhaustion
than the test via si_mem_available().

This will avoid dom0 running out of memory due to excessive foreign
page mappings especially on ARM and on x86 in PVH mode, as those don't
have a pre-ballooned area which can be used for foreign mappings.

As the normal ballooning suffers from the same problem don't balloon
down more than si_mem_available() pages in one iteration. At the same
time limit the default maximum number of retries.

This is part of XSA-300.

Signed-off-by: Juergen Gross <jgross@suse.com>
(cherry picked from commit a1078e821b605813b63bf6bca414a85f804d5c66)

Orabug: 30073695

CVE has not been assigned yet.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jack Vogel <jack.vogel@oracle.com>
Reviewed-by: John Haxby <john.haxby@oracle.com>
Reviewed-by: Patrick Colp <patrick.colp@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>