cmd/link: in TestFallocate, only check number of blocks on Darwin
The number-of-blocks check was introduced when fixing a Darwin-
specific bug. On Darwin, the file allocation syscall is a bit
tricky. On Linux and BSDs, it is more straightforward and unlikely
to go wrong.
The test itself, on the other hand, is less reliable on Linux (and
perhaps BSDs), as it is considered less portable and is an
implementation detail of the file system.
Given these two reasons, only check it on Darwin.
Fixes #75795.
Change-Id: I3da891fd60a141c3eca5d0f5ec20c2cad65b8862
Reviewed-on: https://go-review.googlesource.com/c/go/+/711095
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Keith Randall <khr@golang.org>