xen/gnttab: Fold grant_table_{create,set_limits}() into grant_table_init()

xen/gnttab: Fold grant_table_{create,set_limits}() into grant_table_init()

Now that the max_{grant,maptrack}_frames are specified from the very beginning
of grant table construction, the various initialisation functions can be
folded together and simplified as a result.

Leave grant_table_init() as the public interface, which is more consistent
with other subsystems.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>