zdestroy() patch

[Nicolas Thery]

Hello,

I know that zalloc() is deprecated but here is anyway a patch that
implements zdestroy() as suggested in:

http://leaf.dragonflybsd.org/mailarchive/bugs/2008-01/msg00032.html

This patch is not thread-safe and assumes that the mp lock is held
while zalloc() and friends are called as stated in:

http://leaf.dragonflybsd.org/mailarchive/kernel/2008-01/msg00045.html

I'll commit it in a few days if there are no objections.

Regarding zalloc() lack of thread safety, I see two options:

1/ Modify the man page and comments in vm_zone.c which currently state
the contrary.

2/ Be defensive and add calls to get_mplock() / release_mplock() in
zalloc() and friends.

In any case, the misleading spinlock (zlock) could be removed too.

I've got a preference for (2).  What do you think?

Cheers,
Nicolas
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