LU-12355 lnet: ib_fmr_pool_unmap returns void

LU-12355 lnet: ib_fmr_pool_unmap returns void

Historically ib_fmr_pool_unmap only ever returned 0
Linux kernel 4.20 changed the return for ib_fmr_pool_unmap to void.

Linux-commit: 3eeeb7a59acddaa326b03efdf6dce61c120449a3

Test-Parameters: trivial
Signed-off-by: Shaun Tancheff <stancheff@cray.com>
Change-Id: I49d91a49c452dad5c7d9b153fdbc011f2f25743a
Reviewed-on: https://review.whamcloud.com/35017
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <pkoutoupis@cray.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>