tcp: add LINUX_MIB_PAWS_TW_REJECTED counter

tcp: add LINUX_MIB_PAWS_TW_REJECTED counter

When TCP is in TIME_WAIT state, PAWS verification uses
LINUX_PAWSESTABREJECTED, which is ambiguous and cannot be distinguished
from other PAWS verification processes.

We added a new counter, like the existing PAWS_OLD_ACK one.

Also we update the doc with previously missing PAWS_OLD_ACK.

usage:
'''
nstat -az | grep PAWSTimewait
TcpExtPAWSTimewait              1                  0.0
'''

Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250409112614.16153-3-jiayuan.chen@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>