MFV r328251: 8652 Tautological comparisons with ZPROP_INVAL

MFV r328251: 8652 Tautological comparisons with ZPROP_INVAL

illumos/illumos-gate@4ae5f5f06c6c2d1db8167480f7d9e3b5378ba2f2

https://www.illumos.org/issues/8652:
Clang and GCC prefer to use unsigned ints to store enums. With Clang, that
causes tautological comparison warnings when comparing a zfs_prop_t or
zpool_prop_t variable to the macro ZPROP_INVAL. It's likely that error
handling code is being silently removed as a result.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Igor Kozhukhov <igor@dilos.org>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Alan Somers <asomers@gmail.com>