Spinlocks don't have a buslocked unlock and are faster.

On a P4, time to write a 4M file with 4M one-byte-write()s:

Before:
	0.72s user 5.47s system 99% cpu 6.227 total
	0.76s user 5.40s system 100% cpu 6.154 total
	0.77s user 5.38s system 100% cpu 6.146 total

After:
	1.09s user 4.92s system 99% cpu 6.014 total
	0.74s user 5.28s system 99% cpu 6.023 total
	1.03s user 4.97s system 100% cpu 5.991 total