gnttab: limit mapkind()'s iteration count

gnttab: limit mapkind()'s iteration count

There's no need for the function to observe increases of the maptrack
table (which can occur as the maptrack lock isn't being held) - actual
population of maptrack entries is excluded while we're here (by way of
holding the respective grant table lock for writing, while code
populating entries acquires it for reading). Latch the limit ahead of
the loop, allowing for the barrier to move out, too.

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