LU-6401 header: remove assert from interval_set()

LU-6401 header: remove assert from interval_set()

Currently interval_tree.h is used by user land and
kernel space. To unwind libcfs.h from user space
we need to remove LASSERT from the uapi headers.
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.

Change-Id: Iac716f61542eb6e1a6e060e2810e6b7a175b25d9
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-on: http://review.whamcloud.com/22522
Tested-by: Jenkins
Reviewed-by: Frank Zago <fzago@cray.com>
Tested-by: Maloo <hpdd-maloo@intel.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>