barrier: explicitly ignore return values of barrier_place()

The barrier implementation tracks remote states internally. There is no
need to check the return value of any barrier_*() function if the caller
is not interested in the result. The barrier helpers only return the state
of the remote side, which is usually not interesting as later calls to
barrier_sync() will catch this, anyway.

Shut up coverity by explicitly ignoring return values of barrier_place()
if we're not interested in it.