IB/mlx4: Mark user MR as writable if actual virtual memory is writable

To allow rereg_user_mr to modify the MR from read-only to writable without
using get_user_pages again, we needed to define the initial MR as writable.
However, this was originally done unconditionally, without taking into
account the writability of the underlying virtual memory.

As a result, any attempt to register a read-only MR over read-only
virtual memory failed.

To fix this, do not add the writable flag bit when the user virtual memory
is not writable (e.g. const memory).

However, when the underlying memory is NOT writable (and we therefore
do not define the initial MR as writable), the IB core adds a
"force writable" flag to its user-pages request. If this succeeds,
the reg_user_mr caller gets a writable copy of the original pages.

If the user-space caller then does a rereg_user_mr operation to enable
writability, this will succeed. This should not be allowed, since
the original virtual memory was not writable.

Cc: <stable@vger.kernel.org>
Fixes: 9376932d

 ("IB/mlx4_ib: Add support for user MR re-registration")
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>