Fix the concurrent compaction logic in userfaultfd GC

When kernel doesn't support fault retry (introduced in 5.7), we need the
gc-thread to read from termination-page repeatedly until the thread
count reaches 0.

Bug: 160737021
Test: Manual. Older (<5.7) kernels exercise the fix.
Change-Id: I717b8bcefddb9b0012aa1330eabd2488ce179761