Fix volmode=none property behavior at import time

At import time spa_import() calls zvol_create_minors() directly: with
the current implementation we have no way to avoid device node
creation when volmode=none.

Fix this by enforcing volmode=none directly in zvol_alloc().

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6426

tests/zfs-tests/include/libtest.shlib

@@ -3425,7 +3425,7 @@ function swap_setup
 	typeset swapdev=$1
 
 	if is_linux; then
-		log_must mkswap $swapdev > /dev/null 2>&1
+		log_must eval "mkswap $swapdev > /dev/null 2>&1"
 		log_must swapon $swapdev
 	else
 	        log_must swap -a $swapdev