kernel: fix wrong strlcat call

kernel: fix wrong strlcat call

strlcat takes the total buffer size, not the remaining space.
Passing strlen(cmdline) + l caused it to truncate by one byte
when the appended string fit within the buffer.

This is also more consistent with the rest of the file.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23892
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>