runtime: fix check for vacuous page boundary rounding

sysUnused (e.g., madvise MADV_FREE) is only sensible to call on
physical page boundaries, so scavengelist rounds in the bounds of the
region being released to the nearest physical page boundaries.
However, if the region is smaller than a physical page and neither the
start nor end fall on a boundary, then rounding the start up to a page
boundary and the end down to a page boundary will result in end < start.
Currently, we only give up on the region if start == end, so if we
encounter end < start, we'll call madvise with a negative length and
the madvise will fail.

Issue #16644 gives a concrete example of this:

    start = 0x1285ac000
    end   = 0x1285ae000 (1 8K page)

This leads to the rounded values

    start = 0x1285b0000
    end   = 0x1285a0000

which leads to len = -65536.

Fix this by giving up on the region if end <= start, not just if
end == start.

Fixes #16644.

Change-Id: I8300db492dbadc82ac1ad878318b36bcb7c39524
Reviewed-on: https://go-review.googlesource.com/27230
Reviewed-by: Keith Randall <khr@golang.org>