RDMA/netlink: clean up message validity array initializer

The fix in the parent made me look at that function, and react to how
illogical and illegible the array initializer was.

Use named array indexes to make it clearer what is going on, and make
the initializer not depend silently on the exact index numbers.

[ The initializer now also shows an odd inconsistency in the naming:
  note the IWCM vs IWPM..   - Linus ]

Cc: Leon Romanovsky <leonro@mellanox.com>
Cc: Doug Ledford <dledford@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>