staging: lustre: header: remove assert from interval_set()

In the case of interval_tree.h only interval_set()
uses LASSERT which is removed in this patch and
interval_set() instead reports a real error. The
header libcfs.h for interval_tree.h is not needed
anymore so we can just use the standard linux
kernel headers instead.h

Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6401
Reviewed-on: https://review.whamcloud.com/22522
Reviewed-on: https://review.whamcloud.com/24323


Reviewed-by: Frank Zago <fzago@cray.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>