Recently changes were made to the tcp stack to use a macro/function
to set tcp flags. In the process the PUSH bit setting in the fastpath of
rack was broken. This fixes that as well as cleans up a warning that
is occurring when you don't have INVARIANT on (inp used in KASSERT).

We can use the tcp test suite to find this bug the test plan shows the script
that fails due to the missing push bit

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//

--ip_version=ipv4
--tolerance_usecs=50000

0.00 sysctl -w net.inet.tcp.hostcache.purgenow=1
+0.00 sysctl -w net.inet.tcp.syncookies_only=0
+0.00 sysctl -w net.inet.tcp.syncookies=1
+0.00 sysctl -w net.inet.tcp.rfc1323=0
+0.00 sysctl -w net.inet.tcp.sack.enable=0
+0.00 sysctl -w net.inet.tcp.ecn.enable=2
+0.00 sysctl -w net.inet.tcp.recvspace=65536

+0.00 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
+0.00 fcntl(3, F_GETFL) = 0x02 (flags O_RDWR)
+0.00 fcntl(3, F_SETFL, O_RDWR | O_NONBLOCK) = 0
+0.00 setsockopt(3, IPPROTO_TCP, TCP_NOOPT, [1], 4) = 0
+0.00 connect(3, ..., ...) = -1 EINPROGRESS (Operation now in progress)
+0.00 > S 0:0(0) win 65535
+0.05 < S 0:0(0) win 65535 <sackOK,TS val 100 ecr 0>
+0.00 > S. 0:0(0) ack 1 win 65535
+0.00 < . 1:1(0) ack 1 win 65535
+0.00 %{ assert tcpi_state == TCPI_ESTABLISHED }%
+0.00 %{ assert tcpi_options == 0}%
+0.05 write(3, ..., 5360) = 5360
+0.00 > . 1:537(536) ack 1 win 65535
+0.00 > . 537:1073(536) ack 1 win 65535
+0.00 > . 1073:1609(536) ack 1 win 65535
+0.00 > . 1609:2145(536) ack 1 win 65535
+0.00 > . 2145:2681(536) ack 1 win 65535
+0.00 > . 2681:3217(536) ack 1 win 65535
+0.00 > . 3217:3753(536) ack 1 win 65535
+0.00 > . 3753:4289(536) ack 1 win 65535
+0.00 > . 4289:4825(536) ack 1 win 65535
+0.00 > P. 4825:5361(536) ack 1 win 65535

// Validate DupThresh loss recovery works
+0.01 < . 1:1(0) ack 537 win 65535
+0.01 < . 1:1(0) ack 537 win 65535
+0.01 < . 1:1(0) ack 537 win 65535
+0.01 < . 1:1(0) ack 537 win 65535
+0.00 > . 537:1073(536) ack 1 win 65535

// Verify that no SACK block is sent out
+0.00 < . 10:20(10) ack 5361 win 65535
+0.00 > . 5361:5361(0) ack 1 win 65535

+0.01 close(3) = 0