LU-6142 tests: Fix style issues for unlinkmany.c

LU-6142 tests: Fix style issues for unlinkmany.c

This patch fixes issues reported by checkpatch
for file lustre/tests/unlinkmany.c

This patch also updates the usage message to
to print the information about '-d' option.

Test-Parameters: trivial
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Change-Id: Idd107ba7c005e0186bc39fc9bb4fc84691919178
Reviewed-on: https://review.whamcloud.com/36389
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Ben Evans <bevans@cray.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>