posix: Default value for `batch-fsync-delay-usec` should be '0'

Also fixes for failing testcase `./tests/bugs/bug-888174.t`,
which has been failing sporadically for many patches.

Change-Id: Ic7d2c95da5d3126623cec403207afadd449bf950
BUG: 927146
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5620
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>