IB/core: Modify conditional on ucontext existence

Since we allow to call legacy verbs using their extended counterpart,
the check on ucontext has to move up to a common area in case this verb
is ever extended.

Signed-off-by: Eli Cohen <eli@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>