IB/mlx4: Forbid using sysfs to change RoCE pkeys

commit 2b135db3 upstream.

The pkey mapping for RoCE must remain the default mapping:
VFs:
  virtual index 0 = mapped to real index 0 (0xFFFF)
  All others indices: mapped to a real pkey index containing an
                      invalid pkey.
PF:
  virtual index i = real index i.

Don't allow users to change these mappings using files found in
sysfs.

Fixes: c1e7e466

 ('IB/mlx4: Add iov directory in sysfs under the ib device')
Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>