RDMA/verbs: Check existence of function prior to accessing it

Update all the flows to ensure that function pointer exists prior
to accessing it.

This is much safer than checking the uverbs_ex_mask variable, especially
since we know that test isn't working properly and will be removed
in -next.

This prevents a user triggereable oops.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>