LU-11898 libcfs: do not calculate debug_mb if it is set

LU-11898 libcfs: do not calculate debug_mb if it is set

debug_mb is libcfs module parameter. It should be possible to set it
via

modprobe libcfs libcfs_debug_mb=800

or via adding

options libcfs libcfs_debug_mb=800

to modules configuration.

Fixes: 7092309f32 ("LU-8066 libcfs: migrate to debugfs")
Test-Parameters: trivial
Signed-off-by: Vladimir Saveliev <c17830@cray.com>
Cray-bug-id: LUS-6936
Change-Id: I9da51e44a938a312e43b8a0781b49efc197f7ca9
Reviewed-on: https://review.whamcloud.com/34128
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <c17826@cray.com>