erofs-utils: mkfs: enable `--blobdev` for compressed inode layouts

Add metadata-only image support for compressed inode layouts, e.g.:
 $ mkfs.erofs -zlz4 --blobdev blob.erofs fsmeta.erofs foo/

Note that `-Efragments` and `-Ededupe` haven't supported yet.

Follow-up: support rebuild mode for compressed inode layouts.

Link: https://patch.msgid.link/20260911102852.173556-2-xiang@kernel.org
[ Gao Xiang: "if (!e->inlined && e->plen)" should be "if (e->plen)". ]
Signed-off-by: Gao Xiang <xiang@kernel.org>