maildir: fix maildir folder listing

Previous change "maildir: hide invalid folders" prevents filepath.Walk
from descending into directories that don't contain new/, tmp/ or cur/
because the WalkDirFunc returns filepath.SkipDir.

We have to assume that these directories can be the parent of valid
maildir folders even if they're not themselves valid maildir folders, so
it is a mistake not to recurse into these subfolders. Fix it by
returning nil instead of filepath.SkipDir in the WalkDirFunc.

Fixes: 150aa0f498 ("maildir: hide invalid folders")
Signed-off-by: Julian Pidancet <julian.pidancet@oracle.com>
Acked-by: Robin Jarry <robin@jarry.cc>

worker/maildir/container.go

@@ -81,7 +81,7 @@ func (c *Container) ListFolders() ([]string, error) {
 		// Drop dirs that lack {new,tmp,cur} subdirs
 		for _, sub := range []string{"new", "tmp", "cur"} {
 			if _, err := os.Stat(filepath.Join(path, sub)); os.IsNotExist(err) {
-				return filepath.SkipDir
+				return nil
 			}
 		}