LU-14798 lnet: add LNet GPU Direct Support

LU-14798 lnet: add LNet GPU Direct Support

This patch exports registration/unregistration functions
which are called by the NVFS module to let the LND know
that it can call into the NVFS module to do RDMA mapping
of GPU shadow pages.

GPU priority is considered during NI selection.

Less than 4K writes are always RDMAed if the rdma source is
the gpu device

Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I2bfdbdd5fe3b8536e616ab442d18deace6756d57
lustre-change: https://review.whamcloud.com/37368
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Whamcloud-bug-id: EX-773
Reviewed-on: https://review.whamcloud.com/44110
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>