stand: For zfs, set dv_fmtdev to zfs_fmtdev

Add a generic way to get the string representation of a zfs device / mount.

Sponsored by:		Netflix
Reviewed by:		tsoome (prior version)
Differential Revision:	https://reviews.freebsd.org/D35923

stand/libsa/zfs/Makefile.inc

@@ -60,7 +60,8 @@ CFLAGS_EARLY.zfs.c += ${ZFS_EARLY}
 # zfs.c is special: we need to define HAS_ZSTD_ZFS to get zfssubr.c to initialize zstd
 # properly. We need to have the cddl boot compat directory in the include path for zfssubr.c
 # to be found, and we need a couple of other include paths for skein and lz4. Finally we
-# temporarily need LDRSRC to pick up disk.h until that layering violation is corrected.
+# temporarily need LDRSRC to pick up part.h until libsa has a way to look into partitions
+# or enumerate them...
 #
 CFLAGS.zfs.c+=	-DHAS_ZSTD_ZFS					\
 		-I${SYSDIR}/cddl/boot/zfs			\

stand/libsa/zfs/zfs.c

@@ -1634,6 +1634,7 @@ struct devsw zfs_dev = {
 	.dv_ioctl = noioctl,
 	.dv_print = zfs_dev_print,
 	.dv_cleanup = nullsys,
+	.dv_fmtdev = zfs_fmtdev,
 };
 
 int