mds: do not dump multiple JSON obj

mds: do not dump multiple JSON obj

As enqueue_scrub is invoked twice to perform stray evaluation
at root using new op scrub_mdsdir, tests fail as command
scrub start / scrub_mdsdir would return two JSON object which
JSONDecodeError cannot parse, this patch would make sure we do
not dump JSON for recursive scrub on ~mdsdir than happens along with
scrubbing root (only when using scrub_mdsdir op)

Signed-off-by: Dhairya Parmar <dparmar@redhat.com>
(cherry picked from commit 4a976b5b0110c787b9a8e39360fd41164355036d)