IB/core: Introduce GID entry reference counts

In order to be able to expose pointers to the ib_gid_attrs in the GID
table we need to make it so the value of the pointer cannot be
changed. Thus each GID table entry gets a unique piece of kref'd memory
that is written only during initialization and remains constant for its
lifetime.

This eventually will allow the struct ib_gid_attrs to be returned without
copy from many of query the APIs, but it also provides a way to track when
all users of a HW table index go away.

For roce we no longer allow an in-use HW table index to be re-used for a
new an different entry. When a GID table entry needs to be removed it is
hidden from the find API, but remains as a valid HW index and all
ib_gid_attr points remain valid. The HW index is not relased until all
users put the kref.

Later patches will broadly replace the use of the sgid_index integer with
the kref'd structure.

Ultimately this will prevent security problems where the OS changes the
properties of a HW GID table entry while an active user object is still
using the entry.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>