LU-14428 libcfs: discard cfs_trace_copyin_string()

LU-14428 libcfs: discard cfs_trace_copyin_string()

Instead of cfs_trace_copyin_string(), use memdup_user_nul().
This combines the allocation with the copyin, and nul-terminates.

The resulting code is a lot simpler.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I089c5da96b59ec62d177aea2f3d170bf751c6fec
Reviewed-on: https://review.whamcloud.com/41490
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>