Switch `so_snd' of udp(4) sockets to the new locking scheme.
udp_send() and following udp{,6}_output() do not append packets to
`so_snd' socket buffer. This mean the sosend() and sosplice() sending
paths are dummy pru_send() and there is no problems to simultaneously
run them on the same socket.
Push shared solock() deep down to sesend() and take it only around
pru_send(), but keep somove() running unedr exclusive solock(). Since
sosend() doesn't modify `so_snd' the unlocked `so_snd' space checks
within somove() are safe. Corresponding `sb_state' and `sb_flags'
modifications are protected by `sb_mtx' mutex(9).
Tested and OK bluhm.
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