ANDROID: lkdtm/usercopy: Use __va_function to find proper vm_mmap address

With CFI enabled compiler replacing the reference to vm_mmap
passed to copy_to_user with a pointer to the CFI jump table
in the module itself. So use the __va_function API for
finding the proper address of vm_mmap.

Bug: 166220312
Suggested-by: Sami Tolvanen <samitolvanen@google.com>
Change-Id: Id5bd96cc9bfa1aa99f34bd4785792e0a4eeae5b1
Git-commit: 33752ec915fb9dcccfe3afe4c4df4e032a0d823f
Git-repo: https://android.googlesource.com/kernel/common/


Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>