nspawn: EROFS for chowning mount points is not fatal (#7122)

This fixes --read-only with --private-users. mkdir_userns_p may return
-EROFS if either mkdir or lchown fails; lchown failing is fine as the
mount point will just be overmounted, and if mkdir fails then the
following mount() will also fail (with ENOENT).

(cherry picked from commit 4f13e534)
(cherry picked from commit 1b7014df)