net: pktgen: fix access outside of user given buffer in pktgen_thread_write()

Honour the user given buffer size for the strn_len() calls (otherwise
strn_len() will access memory outside of the user given buffer).

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250219084527.20488-8-ps.report@gmx.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>