crostini: Block suspend while unmounting sshfs

This change ensures that the attempt to unmount crostini's sshfs
completes before the device is allowed to suspend. Previously, the
device may suspend while the unmount attempt is ongoing.

BUG=968060
TEST=Manually test that sshfs is unmounted and device is allowed to \
suspend.

(cherry picked from commit dfdadaaf4053dc6bd7d53f16e2dff522f1e884d0)

Change-Id: Ifd480c2bf6ad90cb1962786e34b2cc359f1e10f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1719777
Commit-Queue: Abhishek Bhardwaj <abhishekbh@chromium.org>
Auto-Submit: Abhishek Bhardwaj <abhishekbh@chromium.org>
Reviewed-by: Joel Hockey <joelhockey@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#681456}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1725310
Cr-Commit-Position: refs/branch-heads/3865@{#31}
Cr-Branched-From: 0cdcc6158160790658d1f033d3db873603250124-refs/heads/master@{#681094}
5 files changed