Commit 8dda1c4c changed the meaning of "nfree" in sweep from the number
of newly freed objects to the total number of free objects in the
span, but didn't update where sweep added nfree to c.local_nsmallfree.
Hence, we're over-accounting the number of frees. This is causing
TestArrayHash to fail with "too many allocs NNN - hash not balanced".

Fix this by computing the number of newly freed objects and adding
that to c.local_nsmallfree, so it behaves like it used to. Computing
this requires a small tweak to mallocgc: apparently we've never set
s.allocCount when allocating a large object; fix this by setting it to
1 so sweep doesn't get confused.

Change-Id: I31902ffd310110da4ffd807c5c06f1117b872dc8
Reviewed-on: https://go-review.googlesource.com/22595Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>