Truncates can take a very long time.  Especially if there is a lot of
writeout happening, because truncate must wait on in-progress I/O.

And sys_unlink() is performing that truncate while holding the parent
directory's i_sem.  This basically shuts down new accesses to the entire
directory until the synchronous I/O completes.

In the testing I've been doing, that directory is /tmp, and this hurts.

So change sys_unlink() to perform the actual truncate outside i_sem.

When there is a continuous streaming write to the same disk, this patch
reduces the time for `make -j4 bzImage' from 370 seconds to 220.