IB/mlx4: fix improper return value

If uhw->inlen is non-zero, the value of variable err is 0 if the copy
succeeds. Then, if kzalloc() or kmalloc() returns a NULL pointer, it
will return 0 to the callers. As a result, the callers cannot detect the
errors. This patch fixes the bug, assign "-ENOMEM" to err before the
NULL pointer checks, and remove the initialization of err at the
beginning.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=189031


Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>