IB/core: Save QP in ib_flow structure

When we create flow steering rule, we need to save the related QP in the
ib_flow struct. this QP is used in destroy flow.

Move the QP assignment from ib_uverbs_ex_create_flow into ib_create_flow,
this would allow both kernel and userspace consumers to use it.

This bug wasn't seen in the wild because there are no kernel consumers
currently in the kernel.

Fixes: 319a441d ("IB/core: Add receive flow steering support")
Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>