udev/spawn: continue to read stdout even if the result buffer is full

udev/spawn: continue to read stdout even if the result buffer is full

Previously, when the stdout of a spawned process (e.g. dmi_memory_id) is
truncated, the event source was not re-enabled, that will cause the process
to remain in a write-blocked state if the stdout buffer is full, and the
process will time out:
```
Spawned process 'dmi_memory_id' [1116] timed out after 2min 59s, killing.
Process 'dmi_memory_id' terminated by signal KILL.
```

The solution is to continue enabling the event source so that on_spawn_io()
can continue reading the stdout buffer. When the result buffer is full, the
local `buf` variable will be used to drain remaining stdout.

Co-authored-by: Deli Zhang <deli.zhang@cloud.com>