LU-14550 libcfs: fix setting of debug_path

LU-14550 libcfs: fix setting of debug_path

While it was possible to set "lctl set_param debug_path=path" or
"echo path > /sys/module/libcfs/parameters/libcfs_debug_file_path"
this change does not affect the path used to dump debug logs.

Connect these parameters to the pathname used for the debug log.

Test-Parameters: testlist=sanity env=ONLY=60f,ONLY_REPEAT=30
Fixes: 7092309f325 ("LU-8066 libcfs: migrate to debugfs")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ic18b5b24d1ac939c09637e66a342f5e3622367c3
Reviewed-on: https://review.whamcloud.com/43109
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>