modprobe zfs during dracut mount

Resolves importing root pool during boot in dracut.  This case was
inadvertently broken with the module autoloading change in #7287.

Reviewed-by: Matthew Thode <prometheanfire@gentoo.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Kash Pande <kash@tripleback.net>
Closes #7322

contrib/dracut/90zfs/mount-zfs.sh.in

@@ -34,6 +34,7 @@ info "ZFS: No sysroot.mount exists or zfs-generator did not extend it."
 info "ZFS: Mounting root with the traditional mount-zfs.sh instead."
 
 # Delay until all required block devices are present.
+modprobe zfs 2>/dev/null
 udevadm settle
 
 if [ "${root}" = "zfs:AUTO" ] ; then