FreeBSD: Add option to rewind checkpoint while importing root pool

This option is used by FreeBSD boot loader.

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mariusz Zaborski <oshogbo@vexillium.org>
Closes openzfs#10738

include/os/freebsd/zfs/sys/zfs_context_os.h

@@ -87,5 +87,5 @@ typedef int fstrans_cookie_t;
 #define	cv_wait_io(cv, mp)			cv_wait(cv, mp)
 typedef struct opensolaris_utsname	utsname_t;
 extern utsname_t *utsname(void);
-extern int spa_import_rootpool(const char *name);
+extern int spa_import_rootpool(const char *name, bool checkpointrewind);
 #endif

module/os/freebsd/zfs/spa_os.c

@@ -185,7 +185,7 @@ spa_generate_rootconf(const char *name)
 }
 
 int
-spa_import_rootpool(const char *name)
+spa_import_rootpool(const char *name, bool checkpointrewind)
 {
 	spa_t *spa;
 	vdev_t *rvd;
@@ -244,6 +244,9 @@ spa_import_rootpool(const char *name)
 	}
 	spa->spa_is_root = B_TRUE;
 	spa->spa_import_flags = ZFS_IMPORT_VERBATIM;
+	if (checkpointrewind) {
+		spa->spa_import_flags |= ZFS_IMPORT_CHECKPOINT;
+	}
 
 	/*
 	 * Build up a vdev tree based on the boot device's label config.

module/os/freebsd/zfs/zfs_vfsops.c

@@ -1580,7 +1580,7 @@ zfs_mount(vfs_t *vfsp)
 
 		error = getpoolname(osname, pname);
 		if (error == 0)
-			error = spa_import_rootpool(pname);
+			error = spa_import_rootpool(pname, false);
 		if (error)
 			goto out;
 	}