Only truncate file types that can be truncated on minixfs.

Only truncate file types that can be truncated on minixfs.

Only regular files, directories and symbolic links can have any
blocks allocated to them - other types of files have different
metadata in their inodes and should not get to the truncation
paths. Enforce this in fs/minix/inode.c.

Without this, deleting a block or character device can cause
minix filesystem corruption. Noted by Konstantin Boldyshev.