mtree: Keep mtree files indented with spaces

The mtree tool indents directory entries with 4 spaces.

Reviewed by: imp
Fixes: 9cab9fde5eda ("virtual_oss: Port to base")
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53979

(cherry picked from commit 3d91a746380e81c2089d8beaae28438867ae6dbf)