RDMA/core: avoid uninitialized variable warning in create_udata

As Dan pointed out, the rework I did makes it harder for smatch and other
static checkers to figure out what is going on with the uninitialized
pointers.

By open-coding the call in create_udata(), we make it more readable for
both humans and tools.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 12f72772 ("IB/uverbs: clean up INIT_UDATA_BUF_OR_NULL usage")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>