net: pktgen: enable 'param=value' parsing

Enable more flexible parameters syntax, allowing 'param=value' in
addition to the already supported 'param value' pattern (additional
this gives the skipping '=' in count_trail_chars() a purpose).

Tested with:

	$ echo "min_pkt_size 999" > /proc/net/pktgen/lo\@0
	$ echo "min_pkt_size=999" > /proc/net/pktgen/lo\@0
	$ echo "min_pkt_size =999" > /proc/net/pktgen/lo\@0
	$ echo "min_pkt_size= 999" > /proc/net/pktgen/lo\@0
	$ echo "min_pkt_size = 999" > /proc/net/pktgen/lo\@0

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250219084527.20488-3-ps.report@gmx.net


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