fileMemoryMapDestroy

fn fileMemoryMapDestroy(userdata: ?*anyopaque, mm: *File.MemoryMap) void {
    const ev: *Evented = @ptrCast(@alignCast(userdata));
    _ = ev;
    const memory = mm.memory;
    if (memory.len == 0) return;
    switch (c.errno(c.munmap(memory.ptr, memory.len))) {
        .SUCCESS => {},
        else => |err| if (builtin.mode == .debug)
            std.log.err("failed to unmap {d} bytes at {*}: {t}", .{ memory.len, memory.ptr, err }),
    }
    mm.* = undefined;
}