Fix an off-by-one when setting the first bit exceeding the current bitmap size

When setting bit n (the n+1th bit) of an n-bit vbitmap, we'd miss to
expand the bitmap and thus overflow our buffer and overwrite the first
bit of the next byte in memory.

include/vbm.h

@@ -86,7 +86,7 @@ vbit_set(struct vbitmap *vb, unsigned bit)
 {
 
 	if (bit >= vb->nbits)
-		vbit_expand(vb, bit);
+		vbit_expand(vb, bit + 1);
 	vb->bits[VBITMAP_IDX(bit)] |= VBITMAP_BIT(bit);
 }