RDMA/uverbs: Tidy input validation of ib_uverbs_rereg_mr()

RDMA/uverbs: Tidy input validation of ib_uverbs_rereg_mr()

Unknown flags should be EOPNOTSUPP, only zero flags is EINVAL. Flags is
actually the rereg action to perform.

The checking of the start/hca_va/etc is also redundant and ib_umem_get()
does these checks and returns proper error codes.

Fixes: 7e6edb9b2e0b ("IB/core: Add user MR re-registration support")
Link: https://lore.kernel.org/r/20201130075839.278575-2-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>