LU-11861 obdclass: fix build with debug kernel

LU-11861 obdclass: fix build with debug kernel

Move declaration before usage.

Lustre-change: https://review.whamcloud.com/34030
Lustre-commit: 2dc87bb143e998e25585673b5f0ba7e2f317475e

Test-Parameters: trivial
Signed-off-by: Alexey Lyashkov <c17817@cray.com>
Change-Id: I9a6c451bb5454b1542f0b06041f6938702e20b36
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-on: https://review.whamcloud.com/41897
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>