Disable create-o_ashift

Occasionally observed failure of create-o_ashift due to the test
case not being 100% reliable.  In order to prevent false positives
disable this test case until it can be made reliable.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #6924
Closes #6925

tests/zfs-tests/tests/functional/cli_root/zpool_create/create-o_ashift.ksh

@@ -42,6 +42,11 @@
 
 verify_runnable "global"
 
+# See issue: https://github.com/zfsonlinux/zfs/issues/6924
+if is_linux; then
+	log_unsupported "Test case occasionally fails"
+fi
+
 function cleanup
 {
 	poolexists $TESTPOOL && destroy_pool $TESTPOOL