erofs-utils: mkfs: support rebuild mode for compressed inodes

Add support for generating a meta-only multidev manifest image with
an overlayfs-like merged tree from multiple compressed EROFS images:

 $ mkfs.erofs -zlz4 -Enoinline_data layerA.erofs layerA/
 $ mkfs.erofs -zlz4 -Enoinline_data layerB.erofs layerB/
 $ mkfs.erofs -zlz4 fsmeta.erofs layerA.erofs layerB.erofs
 # mount -t erofs \
	-odevice=layerA.erofs,device=layerB.erofs
	fsmeta.erofs mnt/

Note that `-Efragments` is not yet supported: it needs further
userspace development.

Cc: Jonathan Calmels <jcalmels@nvidia.com>
Link: https://patch.msgid.link/20260911122110.189175-1-xiang@kernel.org
Signed-off-by: Gao Xiang <xiang@kernel.org>