linux: Unmap the VDSO page when unloading

[markj]

Summary

linux_shared_page_init() creates an object and grabs and maps a single
page to back the VDSO. When destroying the VDSO object, we failed to
destroy the mapping and free KVA.

Test Plan

Load and unload linux.ko and linux64.ko.

Noticed when working on Jeff's vm_object patches.

[markmi_dsl-only.net]

https://ci.freebsd.org/job/FreeBSD-main-i386-build/19172/console shows:

13:07:49 --- all_subdir_linux ---
13:07:49 /usr/src/sys/i386/linux/linux_sysvec.c:941:54: error: too few arguments to function call, expected 2, have 1
13:07:49 __elfN(linux_shared_page_fini)(linux_shared_page_obj);
13:07:49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^

[markj]

In D28696#642861, @markmi_dsl-only.net wrote:

https://ci.freebsd.org/job/FreeBSD-main-i386-build/19172/console shows:

13:07:49 --- all_subdir_linux ---
13:07:49 /usr/src/sys/i386/linux/linux_sysvec.c:941:54: error: too few arguments to function call, expected 2, have 1
13:07:49 __elfN(linux_shared_page_fini)(linux_shared_page_obj);
13:07:49 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^

Thanks, fixed.