linuxkpi: Define EBADSLT to 57

This is the same error code as Linux.

As emaste@ noted in the review, FreeBSD defines the following errno
values in `sys/errno.h`:
* 56 is `EISCONN`
* 57 is `ENOTCONN`

Reviewed by:	manu
Approved by:	manu
Differential Revision:	https://reviews.freebsd.org/D37935

sys/compat/linuxkpi/common/include/linux/errno.h

@@ -34,6 +34,7 @@
 #include <sys/errno.h>
 
 #define	EBADRQC		56	/* Bad request code */
+#define	EBADSLT		57	/* Invalid slot */
 
 #define	ECHRNG		EDOM
 #define	ETIME		ETIMEDOUT