IB/uverbs: Free uapi on destroy

Make sure we free struct uverbs_api once we clean the radix tree. It was
allocated by uverbs_alloc_api().

Fixes: 9ed3e5f4 ("IB/uverbs: Build the specs into a radix tree at runtime")
Reported-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>