libceph: fix "Boolean result is used in bitwise operation" warning

This line dates back to 2013, but cppcheck complained because commit
2f713615 ("libceph: move msgr1 protocol implementation to its own
file") moved it.  Add parenthesis to silence the warning.
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

net/ceph/messenger_v1.c

@@ -1100,7 +1100,7 @@ static int read_partial_message(struct ceph_connection *con)
 		if (ret < 0)
 			return ret;
 
-		BUG_ON(!con->in_msg ^ skip);
+		BUG_ON((!con->in_msg) ^ skip);
 		if (skip) {
 			/* skip this message */
 			dout("alloc_msg said skip message\n");