Standardize and fix mm's exe_file handling for kernels >= 4.1.

Standardize and fix mm's exe_file handling for kernels >= 4.1.

* runtime/task_finder_vma.c (stap_find_exe_file): Rewrite
  stap_find_exe_file() to handle kernels >= 4.1.
* runtime/linux/task_finder.c (__stp_get_mm_path): Call the net
  stap_find_exe_file(), which handles locking the mm.
* runtime/linux/task_finder.c (__stp_get_mm_path): Ditto.
* runtime/linux/uprobes-inode.c (stapiu_get_task_inode): Ditto.
* tapset/linux/task.stp (task_exe_file): Ditto.