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: NLeon Romanovsky <leonro@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>