IB/core: Fix removal of default GID cache entry

When deleting a default GID from the cache, its gid_type field is set
to 0.

This could set the gid_type to RoCE v1 for a RoCE v2 default GID,
essentially making it inaccessible to future modifications, since it
is no longer found by find_gid().

This fix preserves the gid_type value for default gids during cache
operations.

Fixes: b39ffa1d ('IB/core: Add gid_type to gid attribute')
Signed-off-by: Aviv Heller <avivh@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>