udp: Make rehash4 independent in udp_lib_rehash()

udp: Make rehash4 independent in udp_lib_rehash()

As discussed in [0], rehash4 could be missed in udp_lib_rehash() when
udp hash4 changes while hash2 doesn't change. This patch fixes this by
moving rehash4 codes out of rehash2 checking, and then rehash2 and
rehash4 are done separately.

By doing this, we no longer need to call rehash4 explicitly in
udp_lib_hash4(), as the rehash callback in __ip4_datagram_connect takes
it. Thus, now udp_lib_hash4() returns directly if the sk is already
hashed.

Note that uhash4 may fail to work under consecutive connect(<dst
address>) calls because rehash() is not called with every connect(). To
overcome this, connect(<AF_UNSPEC>) needs to be called after the next
connect to a new destination.

[0]
https://lore.kernel.org/all/4761e466ab9f7542c68cdc95f248987d127044d2.1733499715.git.pabeni@redhat.com/

Fixes: 78c91ae2c6de ("ipv4/udp: Add 4-tuple hash for connected socket")
Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Philo Lu <lulie@linux.alibaba.com>
Link: https://patch.msgid.link/20250110010810.107145-1-lulie@linux.alibaba.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>