bpf, doc: Correct one wrong value in "Register value tracking"

If we then OR this with 0x40, then the value of 6th bit (0th is first bit)
become known, so the right mask is 0xbf instead of 0xcf.

Signed-off-by: Wang YanQing <udknight@gmail.com>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

Documentation/networking/filter.txt

@@ -1134,7 +1134,7 @@ The verifier's knowledge about the variable offset consists of:
 mask and value; no bit should ever be 1 in both.  For example, if a byte is read
 into a register from memory, the register's top 56 bits are known zero, while
 the low 8 are unknown - which is represented as the tnum (0x0; 0xff).  If we
-then OR this with 0x40, we get (0x40; 0xcf), then if we add 1 we get (0x0;
+then OR this with 0x40, we get (0x40; 0xbf), then if we add 1 we get (0x0;
 0x1ff), because of potential carries.
 Besides arithmetic, the register state can also be updated by conditional
 branches.  For instance, if a SCALAR_VALUE is compared > 8, in the 'true' branch