staging: lustre: uapi: move lustre_cfg_string() to obd_config.c

To perserve the truncate warning move lustre_cfg_string()
to obd_config.c. A identical function was created for
userland. This function was a bit big for a inline function.

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/26966


Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Ben Evans <bevans@cray.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>