Currently the zero value of an mspan is in the "in use" state. This
seems like a bad idea in general. But it's going to wreak havoc when
we make fixalloc zero allocations: even "freed" mspan objects are
still on the allspans list and still get looked at by the garbage
collector. Hence, if we leave the mspan states the way they are,
allocating a span that reuses old memory will temporarily pass that
span (which is visible to GC!) through the "in use" state, which can
cause "unswept span" panics.

Fix all of this by making the zero state "dead".

Updates #17503.

Change-Id: I77c7ac06e297af4b9e6258bc091c37abe102acc3
Reviewed-on: https://go-review.googlesource.com/31367Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>