task_mmu: Reduce excessive indentation in clear_refs_write

task_mmu: Reduce excessive indentation in clear_refs_write

This is a purely cosmetic fix for clear_refs_write(). It removes excessive
indentation by inverting an if statement and adding a goto statement as
suggested by Bjorn Helgaas <bhelgaas@google.com>;. This is to make upcoming
changes to the file more readable.

Cc: linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>;
Cc: Bjorn Helgaas <bhelgaas@google.com>;
Cc: Primiano Tucci <primiano@chromium.org>;
Cc: Petr Cermak <petrcermak@chromium.org>;
Signed-off-by: Petr Cermak <petrcermak@chromium.org>;

BUG=