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