xen/grant-table: correctly initialize grant table version 1

Commit 85ff6acb (xen/granttable: Grant
tables V2 implementation) changed the GREFS_PER_GRANT_FRAME macro from
a constant to a conditional expression. The expression depends on
grant_table_version being appropriately set. Unfortunately, at init
time grant_table_version will be 0. The GREFS_PER_GRANT_FRAME
conditional expression checks for "grant_table_version == 1", and
therefore returns the number of grant references per frame for v2.

This causes gnttab_init() to allocate fewer pages for gnttab_list, as
a frame can old half the number of v2 entries than v1 entries. After
gnttab_resume() is called, grant_table_version is appropriately
set. nr_init_grefs will then be miscalculated and gnttab_free_count
will hold a value larger than the actual number of free gref entries.

If a guest is heavily utilizing improperly initialized v1 grant
tables, memory corruption can occur. One common manifestation is
corruption of the vmalloc list, resulting in a poisoned pointer
derefrence when accessing /proc/meminfo or /proc/vmallocinfo:

[   40.770064] BUG: unable to handle kernel paging request at 0000200200001407
[   40.770083] IP: [<ffffffff811a6fb0>] get_vmalloc_info+0x70/0x110
[   40.770102] PGD 0
[   40.770107] Oops: 0000 [#1] SMP
[   40.770114] CPU 10

This patch introduces a static variable, grefs_per_grant_frame, to
cache the calculated value. gnttab_init() now calls
gnttab_request_version() early so that grant_table_version and
grefs_per_grant_frame can be appropriately set. A few BUG_ON()s have
been added to prevent this type of bug from reoccurring in the future.

Signed-off-by: Matt Wilson <msw@amazon.com>
Reviewed-and-Tested-by: Steven Noonan <snoonan@amazon.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Annie Li <annie.li@oracle.com>
Cc: xen-devel@lists.xen.org
Cc: linux-kernel@vger.kernel.org
Cc: stable@vger.kernel.org # v3.3 and newer
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>