Dedup IOC enum values in libzfs_input_check

Reuse enum value ZFS_IOC_BASE for `('Z' << 8)`.
This is helpful on FreeBSD where ZFS_IOC_BASE has a different value and
`('Z' << 8)` is wrong.

Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9188

tests/zfs-tests/cmd/libzfs_input_check/libzfs_input_check.c

@@ -816,8 +816,8 @@ zfs_ioc_input_tests(const char *pool)
 
 enum zfs_ioc_ref {
 	ZFS_IOC_BASE = ('Z' << 8),
-	LINUX_IOC_BASE = ('Z' << 8) + 0x80,
-	FREEBSD_IOC_BASE = ('Z' << 8) + 0xC0,
+	LINUX_IOC_BASE = ZFS_IOC_BASE + 0x80,
+	FREEBSD_IOC_BASE = ZFS_IOC_BASE + 0xC0,
 };
 
 /*