dissect: fix fsck

dissect: fix fsck

Since f7725647bb41c3398a867f139efe526efe8aa1b3 when dissecting a disk
image we operate with fds to the device nodes in question wherever we
can. This includes when we fork off fsck, where we pass a /proc/self/fd/
path as argument. This only works if we keep that fd open however and
disable O_CLOEXEC on the fd. Hence do so, and fix fsck this way.

(Without this, all fsck will fail, since the fd path is invalid)