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