Inspired by commit f18c34e4 ("lib: Fix strnlen_user() to not touch
memory after specified maximum") upstream.  This version of
strnlen_user(), no longer present upstream, has a similar off-by-one
error.
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Jan Kara <jack@suse.cz>