crush: don't normalize input of crush_ln iteratively

Use __builtin_clz() supported by GCC and Clang to figure out
how many bits we should shift instead of shifting by a bit
in a loop until the value gets normalized. Improves performance
of this function by up to 3x in worst-case scenario and overall
straw2 performance by ~10%.

Reflects ceph.git commit 110de33ca497d94fc4737e5154d3fe781fa84a0a.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/crush/mapper.c

@@ -253,9 +253,15 @@ static __u64 crush_ln(unsigned int xin)
 
 	/* normalize input */
 	iexpon = 15;
-	while (!(x & 0x18000)) {
-		x <<= 1;
-		iexpon--;
+
+	/*
+	 * figure out number of bits we need to shift and
+	 * do it in one step instead of iteratively
+	 */
+	if (!(x & 0x18000)) {
+		int bits = __builtin_clz(x & 0x1FFFF) - 16;
+		x <<= bits;
+		iexpon = 15 - bits;
 	}
 
 	index1 = (x >> 8) << 1;