IB/core: Make function ib_fmr_pool_unmap return void

Since the function always returns 0 make it void.

Reported-by: Håkon Bugge <haakon.bugge@oracle.com>
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Acked-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>