Low: Filesystem: ignore useless broken pipe error from list_mounts function

Apparently some people were witnessing an influx of 'broken pipe' stderr
messages spamming their log files when managing large numbers of
Filesystem resources. This appears to be the result of the 'cut' cli
tool not properly handling SIGPIPE.  It is safe to ignore this error
and filter it out of the stderr output, which is what this patch does.

Resolves: rhbz#1081240