RDMA/uverbs: Check existence of create_flow callback

RDMA/uverbs: Check existence of create_flow callback

In the accepted series "Refactor ib_uverbs_write path", we presented the
roadmap to get rid of uverbs_cmd_mask and uverbs_ex_cmd_mask fields in
favor of simple check of function pointer. So let's put NULL check of
create_flow function callback despite the fact that uverbs_ex_cmd_mask
still exists.

Link: https://www.spinics.net/lists/linux-rdma/msg60753.html
Suggested-by: Michael J Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>