Infrastructure to support verbs extensions

Infrastructure to support verbs extensions

Infrastructure to support extended verbs capabilities in a forward/backward
manner.

Support for extensions is determeind by the provider calling
verbs_register_driver in place of ibv_register_driver.  When
extensions are enabled, ibverbs sets the current alloc_context /
free_context device operations to NULL.  These are used to
indicate that the struct ibv_device may be cast to struct
verbs_device.

With extensions, ibverbs allocates the ibv_context structure
and calls into the provider to initialize it.  The init call
is part of the verbs_device struct.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Tzahi Oved <tzahio@mellanox.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>