s390/bpf: Fix sk_load_byte_msh()

In sk_load_byte_msh() sk_load_byte_slow() is called instead of
sk_load_byte_msh_slow(). Fix this and call the correct function.

Besides of this load only one byte instead of two and fix the comment.

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>