cmd/link: fix size calculation for file space preallocation on darwin

cmd/link: fix size calculation for file space preallocation on darwin

On darwin, we preallocate file storage space with fcntl
F_ALLOCATEALL in F_PEOFPOSMODE mode. This is specified as
allocating from the physical end of the file. So the size we give
it should be the increment, instead of the total size.

Fixes #39044.

Change-Id: I10c7ee8d51f237b4a7604233ac7abc6f91dcd602
Reviewed-on: https://go-review.googlesource.com/c/go/+/234481
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>