erofs-utils: mkfs: strip overlay origin xattrs with --ovlfs-strip

Fix --ovlfs-strip failing to remove trusted.overlay.origin from output
inodes. Also consolidate the opaque and origin xattr helpers.

Fixes: dd40232ee060 ("erofs-utils: mkfs: add `--ovlfs-strip` option")
Fixes: 8286289d1959 ("erofs-utils: lib: avoid duplicate overlay origin xattrs")
Assisted-by: Codex:gpt-6-astra
Signed-off-by: Yifan Zhao <stopire@gmail.com>
Link: https://patch.msgid.link/20261112124714.2774025-1-stopire@gmail.com
Signed-off-by: Gao Xiang <xiang@kernel.org>