Fix write(2) returns zero bug from 933ec99

For generic_write_checks with 2 args, we can exit when it returns zero because
it means count is zero. However this is not the case for generic_write_checks
with 4 args, where zero means no error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Haakan T Johansson <f96hajo@chalmers.se>
Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Closes #5720
Closes #5726