Fixes a race condition where freeing a buffer and immediately re-allocating and
adding it would fail to pass the file descriptor to udebugd
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Can be used to reserve worst case length using udebug_entry_append,
then setting the final length using udebug_entry_set_length
Signed-off-by: Felix Fietkau <nbd@nbd.name>