ALSA: seq: Don't allow resizing pool in use

ALSA: seq: Don't allow resizing pool in use

This is a fix for a (sort of) fallout in the recent commit
d15d662e89fc ("ALSA: seq: Fix racy pool initializations") for
CVE-2018-1000004.
As the pool resize deletes the existing cells, it may lead to a race
when another thread is writing concurrently, eventually resulting a
UAF.

A simple workaround is not to allow the pool resizing when the pool is
in use.  It's an invalid behavior in anyway.

Fixes: d15d662e89fc ("ALSA: seq: Fix racy pool initializations")
Reported-by: 范龙飞 <[email protected]>
Reported-by: Nicolai Stange <[email protected]>
Cc: <[email protected]>
Signed-off-by: Takashi Iwai <[email protected]>