bitmap API (include/linux/bitmap.h) has 'bitmap' prefix for its methods.

On the other hand DM bitmap API is special case.
Adding 'dm' prefix to it to avoid potential name space collision.

No functional changes intended.
Suggested-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NAndy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: NMike Snitzer <snitzer@redhat.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>