LU-6142 lustre: unwrap some ldebugfs_register() calls

LU-6142 lustre: unwrap some ldebugfs_register() calls

When the third option (list) to ldebugfs_register() is NULL, it's the
same as just calling debugfs_create_dir().  So unwind this and call
debugfs_create_dir() directly.

This ends up saving lots of code as we do not need to do any error
checking of the return value (because it does not matter).

Note: this depends on Commit a59d6293e537 ("debugfs: change parameter
check in debugfs_remove() functions") which landed in v3.6-rc1.

Linux-Commit b145f49f233d ("staging: lustre: unwrap some
	ldebugfs_register() calls")

Test-Parameters: trivial
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I6bff3b1d416de1f15bb95b22fa6075b1ae30a534
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Reviewed-on: https://review.whamcloud.com/37742
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>