RDMA/bnxt_re: Validate udata before executing commands

In the Linux kernel, the following vulnerability has been resolved:

RDMA/bnxt_re: Validate udata before executing commands

The destroy callbacks currently zero the udata output after tearing down
driver resources. If the userspace access fails, uverbs preserves the
uobject and allows the destroy callback to run again, even though the
driver resource has already been freed.

Call ib_no_udata_io() before teardown so udata failures are detected
while the resource is still intact, then return success after teardown
completes.

As part of this change, move ib_respond_empty_udata() to the start of
the create and modify flows. While this is not strictly required for
general create flows, as the core layer unwinds uobjects on failure, it
is necessary for create AH. In _rdma_create_ah(), the HW object is
otherwise leaked.