xen/gnttab: Export opt_max_{grant,maptrack}_frames

xen/gnttab: Export opt_max_{grant,maptrack}_frames

This is to facilitate the values being passed in via domain_create(), at which
point the dom0 construction code needs to know them.

While cleaning up, drop the DEFAULT_* defines, which are only used immediately
adjacent in a context which makes it obvious that they are the defaults, and
drop the (unused) logic to allow a per-arch override of
DEFAULT_MAX_NR_GRANT_FRAMES.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>