RDMA/umem: Remove unused 'work' member from struct ib_umem

It is not used now.

Fixes: b95df5e3 ("drivers/IB,core: reduce scope of mmap_sem")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/0-v1-22a2667fa089+a3-umem_work_jgg@nvidia.com


Reviewed-by: Devesh Sharma <devesh.s.sharma@oracle.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>