tcp: remove one ktime_get() from recvmsg() fast path

Each time some payload is consumed by user space (recvmsg() and friends),
TCP calls tcp_rcv_space_adjust() to run DRS algorithm to check
if an increase of sk->sk_rcvbuf is needed.

This function is based on time sampling, and currently calls
tcp_mstamp_refresh(tp), which is a wrapper around ktime_get_ns().

ktime_get_ns() has a high cost on some platforms.
100+ cycles for rdtscp on AMD EPYC Turin for instance.

We do not have to refresh tp->tcp_mpstamp, using the last cached value
is enough. We only need to refresh it from __tcp_cleanup_rbuf()
if an ACK must be sent (this is a rare event).

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20251024120707.3516550-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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