IB: Enable uverbs_destroy_def_handler to be used by drivers

Enable uverbs_destroy_def_handler to be used by drivers and replace
current code to use it.

Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>