Revert "udp: avoid calling sock_def_readable() if possible"

This reverts commit 612b1c0d. On a
scenario with multiple threads blocking on a recvfrom(), we need to call
sock_def_readable() on every __udp_enqueue_schedule_skb() otherwise the
threads won't be woken up as __skb_wait_for_more_packets() is using
prepare_to_wait_exclusive().

Link: https://bugzilla.redhat.com/2308477


Fixes: 612b1c0d ("udp: avoid calling sock_def_readable() if possible")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20241202155620.1719-1-ffmancera@riseup.net


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