RDMA/core: Do not read wild stack memory in uverbs_get_handler_fn()

RDMA/core: Do not read wild stack memory in uverbs_get_handler_fn()

Sashiko points out the legacy write path in ib_uverbs_write() does
allocate a struct uverbs_attr_bundle, but it doesn't wrap it in a
bundle_priv so downcasting here isn't safe.

Instead lift the method_elm out of the bundle_priv and use it for the
debug function. The legacy write path will leave it set as NULL since the
write method_elm uses a different type.

Cc: stable@vger.kernel.org
Fixes: 1de9287ece44 ("RDMA: Add ib_copy_validate_udata_in()")
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>