gnttab/ARM: don't corrupt shared GFN array

gnttab/ARM: don't corrupt shared GFN array

... by writing status GFNs to it. Introduce a second array instead.
Also implement gnttab_status_gmfn() properly now that the information is
suitably being tracked.

While touching it anyway, remove a misguided (but luckily benign) upper
bound check from gnttab_shared_gmfn(): We should never access beyond the
bounds of that array.

This is part of XSA-255.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>