LU-11233 utils: fix build warnings for gcc8

LU-11233 utils: fix build warnings for gcc8

Quiet new build warnings that appear with GCC8, mainly related
to the length of string buffers not being long enough (in theory)
for the maximum possible string sizes, even if this never actually
is possible in practice.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I83a955fc68f3e03fe84622ddf1cedfb30d5916ac
Reviewed-on: https://review.whamcloud.com/34662
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>