gnttab: adjust pin count overflow checks

gnttab: adjust pin count overflow checks

It's at least odd to check counters which aren't going to be
incremented, resulting in failure just because prior operations may have
reached the refcount limit. And it's also not helpful to use open-coded
literal numbers in these checks.

Calculate the increment values first and derive from them the mask to
use in the checks.

Also move the pin count checks ahead of the calculation of the status
(and for copy also sha2) pointers: They're not needed in the failure
cases, and this way the compiler may also have an easier time keeping
the variables at least transiently in registers for the subsequent uses.

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