Set O_NOATIME flag on Linux

Citing Kerrisk, The Linux Programming Interface:

    The O_NOATIME flag is intended for use by indexing and backup
    programs. Its use can significantly reduce the amount of disk
    activity, because repeated disk seeks back and forth across the
    disk are not required to read the contents of a file and to update
    the last access time in the file’s i-node[.]

restic used to do this, but the functionality was removed along with the
fadvise call in #670.

internal/fs/setflags_linux.go

@@ -0,0 +1,21 @@
+package fs
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// SetFlags tries to set the O_NOATIME flag on f, which prevents the kernel
+// from updating the atime on a read call.
+//
+// The call fails when we're not the owner of the file or root. The caller
+// should ignore the error, which is returned for testing only.
+func setFlags(f *os.File) error {
+	fd := f.Fd()
+	flags, err := unix.FcntlInt(fd, unix.F_GETFL, 0)
+	if err == nil {
+		_, err = unix.FcntlInt(fd, unix.F_SETFL, flags|unix.O_NOATIME)
+	}
+	return err
+}