[ Reason ]
Fix #1149535, which documents a deadlock, by applying an upstream commit.

[ Impact ]
The parent waits for lsblk to terminate before consuming stdout. If lsblk
produces enough JSON output to fill the pipe, lsblk blocks waiting for the
parent to consume stdout, while the parent blocks waiting for lsblk to exit.

[ Tests ]
I have only compiled the package and have started xfs_scrub_all once.

[ Risks ]
Code is a 6-line change that gets rid of the wait()

[ Checklist ]
  [x] *all* changes are documented in the d/changelog
  [x] I reviewed all changes and I approve them
  [x] attach debdiff against the package in (old)stable
  [x] the issue is verified as fixed in unstable

[ Changes ]
Applies one upstream commit that was identified by the reporter to fix
the deadlock issue.