FreeBSD: bring back possibility to rewind the checkpoint from bootloader

Add parsing of the rewind options.

When I was upstreaming the change [1], I omitted the part where we
detect that the pool should be rewind. When the FreeBSD repo has
synced with the OpenZFS, this part of the code was removed.

[1] FreeBSD repo: 277f38abffc6a8160b5044128b5b2c620fbb970c
[2] OpenZFS repo: f2c027bd

External-issue: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254152


Originally reviewed by: tsoome, allanjude
Originally reviewed by: kevans (ok from high-level overview)
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mariusz Zaborski <oshogbo@vexillium.org>
Closes #11730 

module/os/freebsd/zfs/zfs_vfsops.c

@@ -1291,6 +1291,18 @@ getpoolname(const char *osname, char *poolname)
 	return (0);
 }
 
+static void
+fetch_osname_options(char *name, bool *checkpointrewind)
+{
+
+	if (name[0] == '!') {
+		*checkpointrewind = true;
+		memmove(name, name + 1, strlen(name));
+	} else {
+		*checkpointrewind = false;
+	}
+}
+
 /*ARGSUSED*/
 static int
 zfs_mount(vfs_t *vfsp)
@@ -1301,6 +1313,7 @@ zfs_mount(vfs_t *vfsp)
 	char		*osname;
 	int		error = 0;
 	int		canwrite;
+	bool		checkpointrewind;
 
 	if (vfs_getopt(vfsp->mnt_optnew, "from", (void **)&osname, NULL))
 		return (SET_ERROR(EINVAL));
@@ -1314,6 +1327,8 @@ zfs_mount(vfs_t *vfsp)
 		secpolicy_fs_mount_clearopts(cr, vfsp);
 	}
 
+	fetch_osname_options(osname, &checkpointrewind);
+
 	/*
 	 * Check for mount privilege?
 	 *
@@ -1392,7 +1407,7 @@ zfs_mount(vfs_t *vfsp)
 
 		error = getpoolname(osname, pname);
 		if (error == 0)
-			error = spa_import_rootpool(pname, false);
+			error = spa_import_rootpool(pname, checkpointrewind);
 		if (error)
 			goto out;
 	}