LU-9943 lnd: correct WR fast reg accounting

LU-9943 lnd: correct WR fast reg accounting

Ensure that enough WRs are allocated for the fast reg
case which needs two additional WRs per transfer:
the first for memory window registration and the second for
memory window invalidation.

Failure to allocate these causes the following problem:
mlx5_warn:mlx5_0:begin_wqe:4085(pid 9590): work queue overflow

Test-Parameters: trivial
Signed-off-by: Alexey Lyashkov <alexey.lyashkov@seagate.com>
Signed-off-by: Amir Shehata <amir.shehata@intel.com>
Change-Id: Icf98b6bbb3d98fb29794173da84412070f13541b
Reviewed-on: https://review.whamcloud.com/30311
Reviewed-by: Alexey Lyashkov <c17817@cray.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Doug Oucharek <dougso@me.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>