RDMA/umem: Refactor exit paths in ib_umem_get

Simplify exit paths in ib_umem_get to use the standard goto unwind
pattern.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>