xen: Fix warning by adding KERNBASE to modlist_paddr before casting

Clang 13 produces the following warning for hammer_time_xen():

sys/x86/xen/pv.c:183:19: error: the pointer incremented by -2147483648 refers past the last possible element for an array in 64-bit address space containing 256-bit (32-byte) elements (max possible 576460752303423488 elements) [-Werror,-Warray-bounds]

(vm_paddr_t)start_info->modlist_paddr + KERNBASE;
            ^                               ~~~~~~~~

sys/xen/interface/arch-x86/hvm/start_info.h:131:5: note: array 'modlist_paddr' declared here

uint64_t modlist_paddr;         /* Physical address of an array of           */
^

This is because the expression first casts start_info->modlist_paddr to
struct hvm_modlist_entry * (via vmpaddr_t), and *then* adds KERNBASE,
which is then interpreted as KERNBASE * sizeof(struct
hvm_modlist_entry).

Instead, parenthesize the addition to get the intended result, and cast
it to struct hvm_modlist_entry * afterwards. Also remove the cast to
vmpaddr_t since it is not necessary.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D31711

(cherry picked from commit 8e3c56d6b676a175e974bad4c20797fb35017db8)