bitvec: Add bitvec_tailroom_bits() function

This is similar to msgb_tailroom(): It returns the amount of space
left at the end of the bit vector (compared to the current cursor).

The function returns the number of bits left in the bitvec.

Change-Id: I8980a6b6d1973b67a2d9ad411c878d956fb428d1
diff --git a/include/osmocom/core/bitvec.h b/include/osmocom/core/bitvec.h
index da2d4e4..bd10709 100644
--- a/include/osmocom/core/bitvec.h
+++ b/include/osmocom/core/bitvec.h
@@ -93,4 +93,10 @@
 	return bytes;
 }
 
+/*! Return the tailroom in number of unused bits remaining in the bit-vector */
+static inline unsigned int bitvec_tailroom_bits(const struct bitvec *bv)
+{
+	return bv->data_len*8 - bv->cur_bit;
+}
+
 /*! @} */