adaptive serialize enter

[Sepherosa Ziehau]

Hi,

Following patch adds adaptive serialize enter support for SMP.  It
spins a little bit before calling tsleep.  I have used it on ifq in a
local patch with very good results.  In the current setting, it favors
spinning more than sleeping.  debug.serialize_boround and
debug.serialize_bolimit could be used to change what it favors.  When
the contention is too heavy, it is much better than pure spinlock;
system will not be choked for ~1seconds under extreme condition by
chance (it is really very _rare_ after randomizing spinlock backoff,
but it does happen)

Please review it:
http://leaf.dragonflybsd.org/~sephe/lwkt_serialize.c.diff

Best Regards,
sephe

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