Avoid holding semaphore while making mmap callbacks.

Avoid holding semaphore while making mmap callbacks.
* runtime/task_finder.c (__stp_call_mmap_callbacks_for_task): Grabs the
  'mmap_sem' sempaphore. Caches vma information, releases the semaphore,
  then makes mmap callbacks.
  (__stp_utrace_task_finder_target_quiesce): Calls
  __stp_call_mmap_callbacks_for_task() to make mmap callbacks on initial
  attach to a task.