staging: lustre: libcfs: move add_wait_queue_exclusive_head to lustre layer

staging: lustre: libcfs: move add_wait_queue_exclusive_head to lustre layer

Only lustre client uses add_wait_queue_exclusive_head() so move
it from libcfs layer to lustre_lib.h where it is needed.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6245
Reviewed-on: http://review.whamcloud.com/13874
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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>