linuxkpi: Update struct vm_area_struct when an existing mapping is extended

With Mesa 26 and DRM drivers in Linux 6.13, userspace will try to extend
an existing mmap, at last push the end address further.

Before this change, the mapping was not updated, but userspace would try
to access a page after the initial end address, leading to a panic
triggered by the following assertion in vm_fault_populate():

MPASS(fs->first_pindex <= pager_last);

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D58195