sx: fix adaptive spinning broken in r327397

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.
      .-'---`-.			      .-'---`-.
    ,'          `.		    ,'          `.
    |             \		    |             \
    |              \		    |              \
    \           _  \		    \           _  \
    ,\  _    ,'-,/-)\		    ,\  _    ,'-,/-)\
    ( * \ \,' ,' ,'-)		    ( * \ \,' ,' ,'-)
     `._,)     -',-')		     `._,)     -',-')
       \/         ''/		       \/         ''/
        )        / /		        )        / /
       /       ,'-'		       /       ,'-'