LU-12392 utils: specify correct size for the buffer

LU-12392 utils: specify correct size for the buffer

otherwise gcc8 makes a warning which interrupts build.

Change-Id: I6a94c6cd63473df9fc88b1867bbda1353fa10247
Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35070
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>