LU-6401 headers: Create single .h for lu_seq_range

Put lu_seq_range related functions into a single .h.
Include directly from files which use it, and remove definitions from
lustre_idl.h

Signed-off-by: Ben Evans <bevans@cray.com>
Change-Id: I2e8912688bc5b9fd77fd7b1c85d2dee27cfd825c
Reviewed-on: http://review.whamcloud.com/15952
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>