Also fix memFile.Readdir(n) to return nil error when n <= 0, to match
package os. Fix memFile.Write to fail when writing to directories. Avoid
taking the node lock twice for memFile methods.

Change-Id: I72b0753c9376c3889972662e0454efe67d73479a
Reviewed-on: https://go-review.googlesource.com/2711Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Nick Cooper <nmvc@google.com>