cmd/link: fix mode parameter to fallocate on Linux

cmd/link: fix mode parameter to fallocate on Linux

Fix the mode parameter to fallocate on Linux which is the operation mode
and not the file mode as with os.OpenFile.

Also handle syscall.EINTR.

Fixes #38950

Change-Id: Ieed20d9ab5c8a49be51c9f9a42b7263f394a5261
Reviewed-on: https://go-review.googlesource.com/c/go/+/232805
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Jeremy Faller <jeremy@golang.org>