LU-6142 lustre: make ldebugfs_add_vars a void function

LU-6142 lustre: make ldebugfs_add_vars a void function

The call to ldebugfs_add_vars() can not really fail, so have it just
return nothing, which allows us to clean up a lot of unused error
handling code.

Linux-Commit d5bf5cf9e150 ("staging: lustre: make ldebugfs_add_vars a
void function")

Test-Parameters: trivial
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I338c573fe5cb7dbd3595247f5c3647f5c1bf949b
Reviewed-on: https://review.whamcloud.com/37744
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>