staging/lustre: make ldebugfs_remove recursive

ldebugfs_remove is usually called on directories with files passed in
as attributes, so simple debugfs_remove failes on them as not empty
Switch to debugfs_remove_recursive.

This fixes a number of problems where a new filesystem is mounted after
being unmounted first.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>