syscall: copy original rlimit before modifying

CL 531516 converted origRlimitNofile from an atomic.Value to
atomic.Pointer[Rlimit]. i.e., it changed from storing a value to storing
a pointer.

After storing a pointer to lim, the remainder of this function
immediately modifies it, thus mutating the value pointer to by
origRlimitNofile (and thus defeating the point of origRlimitNofile).

This broke the android-amd64-emu builder because it is (apparently) the
only builder where the original RLIMIT_NOFILE Cur != Max.
TestRlimitRestored is skipped on every other builder.

Change-Id: I12076350eeddfd221823ad651e7e7eca59d2bdcd
Reviewed-on: https://go-review.googlesource.com/c/go/+/532100
Run-TryBot: Michael Pratt <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: prattmic

src/syscall/rlimit.go

@@ -31,9 +31,10 @@ func init() {
 	var lim Rlimit
 	if err := Getrlimit(RLIMIT_NOFILE, &lim); err == nil && lim.Cur != lim.Max {
 		origRlimitNofile.Store(&lim)
-		lim.Cur = lim.Max
-		adjustFileLimit(&lim)
-		setrlimit(RLIMIT_NOFILE, &lim)
+		nlim := lim
+		nlim.Cur = nlim.Max
+		adjustFileLimit(&nlim)
+		setrlimit(RLIMIT_NOFILE, &nlim)
 	}
 }