x86/xen: Use DIV_ROUND_UP

x86/xen: Use DIV_ROUND_UP

The kernel.h macro DIV_ROUND_UP performs the computation
(((n) + (d) - 1) /(d)) but is perhaps more readable.

The Coccinelle script used to make this change is as follows:
@haskernel@
@@

@depends on haskernel@
expression n,d;
@@

(
- (n + d - 1) / d
+ DIV_ROUND_UP(n,d)
|
- (n + (d - 1)) / d
+ DIV_ROUND_UP(n,d)
)

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 585423c8c4d2f39a2c299bc6dd16433e6141fba5)
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Orabug: 24820937