dracut: make module-setup.sh shebang explicit

while these are source by dracut (which is a bash script)
the practical difference is small, but it is more correct:

/bin/sh is not bash on all systems (e.g. Debian and its
derivatives use /bin/dash as /bin/sh by default).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
Closes #6491

contrib/dracut/02zfsexpandknowledge/module-setup.sh.in

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 get_devtype() {
   local typ

contrib/dracut/90zfs/module-setup.sh.in

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 check() {
 	# We depend on udev-rules being loaded