Fix zpl_mknod() return values

The zpl_mknod() function was incorrectly negating its return value.
This doesn't cause any problems in the success case, but it does
prevent us from returning the correct error code for a failure.
The implementation of this function is now consistent with all
the other zpl_* functions.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1717