Add bootfs.snapshot and bootfs.rollback kernel parameters

Unlike other filesystems, snapshots and rollbacks of bootfs need to be
done from a rescue environment. This patch makes it possible to snap-
shot or rollback the bootfs simply by specifying bootfs.snapshot or
bootfs.rollback on the kernel command line. The operation will be
performed by dracut just before bootfs is mounted.

Reviewed-by: Antonio Russo <antonio.e.russo@gmail.com> 
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Gregory Bartholomew <gregory.lee.bartholomew@gmail.com>
Closes #10198

contrib/dracut/90zfs/zfs-rollback-bootfs.service.in

@@ -0,0 +1,14 @@
+[Unit]
+Description=Rollback bootfs just before it is mounted
+Requisite=zfs-import.target
+After=zfs-import.target zfs-snapshot-bootfs.service
+Before=dracut-mount.service
+DefaultDependencies=no
+ConditionKernelCommandLine=bootfs.rollback
+
+[Service]
+# ${BOOTFS} should have been set by zfs-env-bootfs.service
+Type=oneshot
+ExecStartPre=/bin/sh -c 'test -n "${BOOTFS}"'
+ExecStart=/bin/sh -c '. /lib/dracut-lib.sh; SNAPNAME="$(getarg bootfs.rollback)"; /sbin/zfs rollback -Rf "${BOOTFS}@${SNAPNAME:-%v}"'
+RemainAfterExit=yes