sx: fix adaptive spinning broken in r327397
The condition was flipped.
In particular heavy multithreaded kernel builds on zfs started suffering
due to nested sx locks.
For instance make -s -j 128 buildkernel:
before: 3326.67s user 1269.62s system 6981% cpu 1:05.84 total
after: 3365.55s user 911.27s system 6871% cpu 1:02.24 total
ps.
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