s390/lcs: Convert sprintf to scnprintf

This LWN article explains the why scnprintf is preferred over snprintf
in general
https://lwn.net/Articles/69419/


Ie. snprintf() returns what *would* be the resulting length, while
scnprintf() returns the actual length.

Reported-by: Jules Irenge <jbi.octave@gmail.com>
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Thorsten Winkler <twinkler@linux.ibm.com>
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>