glusterfsd: use-readdirp w/o arguments should not turn off readdirp

`use-readdirp` has an optional argument in argp -
specifying just `--use-readdirp` command line should not
'turn off' readdirp, since that undermines the meaning
of such an argument.

Change-Id: I965d87e29bd0d61997d9be96fa698e270a2ee173
BUG: 983477
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/5851
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>