fs/squashfs: Fix index off by 1 for inode SQFS_LDIR_TYPE

fs/squashfs: Fix index off by 1 for inode SQFS_LDIR_TYPE

I've created a squashfs file system with Yocto (it use squashfs-tools)
and u-boot command sqfsls give the error:'Error while searching inode:
unknown type.'
After some digging in the code I found that the index is off by 1.
This patch fix this issue and I can successful use the sqfsls command.
After search for the squashfs format I found a link talk about a
similar issue but this time in the documentation. The link is:
https://github.com/AgentD/squashfs-tools-ng/commit/e6588526838caece9529

Signed-off-by: Gerard Koskamp <gerard.koskamp@nedap.com>
Tested-by: Joao Marcos Costa <joaomarcos.costa@bootlin.com>