[PATCH] convert file_lock to a spinlock

[PATCH] convert file_lock to a spinlock

Time to write a 2M file, one byte at a time:

Before:
        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

After:
	0.79s user 5.17s system 99% cpu 5.993 total
	0.79s user 5.17s system 100% cpu 5.957 total
	0.84s user 5.11s system 100% cpu 5.942 total