Switch to bitmap_zalloc() to show clearly what we are allocating.  Besides
that it returns pointer of bitmap type ("unsigned long *") instead of the
opaque "void *".
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>