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

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

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

This patch also removes locally defined difftime
macro with C library defined difftime function

Test-Parameters: trivial testlist=recovery-small
Signed-off-by: Arshad Hussain <arshad.super@gmail.com>
Change-Id: I10458cd1467639ba613b303f2fbe8cd24efc9942
Reviewed-on: https://review.whamcloud.com/35906
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>