IB/uverbs: Fix ordering of ucontext check in ib_uverbs_write

During disassociation the ucontext will become NULL, however due to how
the SRCU locking works the ucontext must only be examined after looking
at the ib_dev, which governs the RCU control flow.

With the wrong ordering userspace will see EINVAL instead of EIO for a
disassociated uverbs FD, which breaks rdma-core.

Cc: stable@vger.kernel.org
Fixes: 491d5c6a

 ("RDMA/uverbs: Move uncontext check before SRCU read lock")
Reported-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>