Skip slow tests when kmemleak is enabled

When running the ZFS Test Suite with a kmemleak enabled kernel
the following test cases run far slower than usual and may hit
their timeout threshold.  Skip the following test cases.

Test: cli_root/zfs_get/zfs_get_009_pos (run as root) [55:43]
Test: cli_root/zpool_clear/zpool_clear_001_pos (run as root) [11:32]
Test: cli_root/zpool_create/zpool_create_024_pos (run as root) [11:01]
Test: features/async_destroy/async_destroy_001_pos (run as root) [41:15]
Test: inheritance/inherit_001_pos (run as root) [09:08]

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5479 
Closes #5480

tests/zfs-tests/include/libtest.shlib

@@ -57,6 +57,19 @@ function is_32bit
 	fi
 }
 
+# Determine if kmemleak is enabled
+#
+# Return 0 if kmemleak is enabled, 1 if otherwise
+
+function is_kmemleak
+{
+	if is_linux && [[ -e /sys/kernel/debug/kmemleak ]]; then
+		return 0
+	else
+		return 1
+	fi
+}
+
 # Determine whether a dataset is mounted
 #
 # $1 dataset name