From fts.c comment:

The "FTS_NOSTAT" option can avoid a lot of calls to stat(2) if it knows that a
directory could not possibly have subdirectories. This is decided by looking at
the link count: a subdirectory would increment its parent's link count by
virtue of its own ".." entry.  This assumption only holds for UFS-like
filesystems that implement links and directories this way, so we must punt for
others.

It looks like ZFS is a UFS-like file system, as the above also holds for ZFS.
Add ZFS to the list of file systems that allow for such optimization.

MFC after:	1 month

lib/libc/gen/fts-compat.c

@@ -110,6 +110,7 @@ struct _fts_private {
 
 static const char *ufslike_filesystems[] = {
 	"ufs",
+	"zfs",
 	"nfs",
 	"nfs4",
 	"ext2fs",

lib/libc/gen/fts.c

@@ -100,6 +100,7 @@ struct _fts_private {
 
 static const char *ufslike_filesystems[] = {
 	"ufs",
+	"zfs",
 	"nfs",
 	"nfs4",
 	"ext2fs",