Currently it triggers a WARN_ON and then goes ahead and destroys the
uobject anyhow, leaking any driver memory.

The only place that leaks driver memory should be during FD close() in
uverbs_destroy_ufile_hw().

Drivers are only allowed to fail destroy uobjects if they guarantee
destroy will eventually succeed. uverbs_destroy_ufile_hw() provides the
loop to give the driver that chance.

Link: https://lore.kernel.org/r/20200902081708.746631-1-leon@kernel.orgSigned-off-by: NLeon Romanovsky <leonro@nvidia.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>