linux/bitmap.h: add bitmap_empty helper

Import this function from Linux.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index 0a8503a..baff77e 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -196,6 +196,14 @@
 	}
 }
 
+static inline bool bitmap_empty(const unsigned long *src, unsigned nbits)
+{
+	if (small_const_nbits(nbits))
+		return ! (*src & BITMAP_LAST_WORD_MASK(nbits));
+
+	return find_first_bit(src, nbits) == nbits;
+}
+
 static inline void bitmap_or(unsigned long *dst, const unsigned long *src1,
 			     const unsigned long *src2, unsigned int nbits)
 {