libspl: declare aok extern in header

Rather than defining a new instance of 'aok' in every compilation
unit which includes this header, there is a single instance
defined in zone.c, and the header now only declares an extern.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Zuchowski <pzuchowski@datto.com>
Signed-off-by: Nick Black <dankamongmen@gmail.com>
Closes #9752

lib/libspl/include/assert.h

@@ -34,7 +34,7 @@
 #include <stdarg.h>
 
 #ifndef _KERNEL
-int aok;
+extern int aok;
 #endif
 
 static inline int

lib/libspl/zone.c

@@ -27,6 +27,8 @@
 #include <string.h>
 #include <errno.h>
 
+int aok = 0;
+
 zoneid_t
 getzoneid()
 {

tests/zfs-tests/tests/functional/checksum/edonr_test.c

@@ -40,6 +40,8 @@
 #include <sys/time.h>
 #include <sys/stdtypes.h>
 
+int aok = 0;
+
 /*
  * Test messages from:
  * http://csrc.nist.gov/groups/ST/toolkit/documents/Examples/SHA_All.pdf