We permanently hold the i_sem of swapfiles so that nobody can addidentally
ftruncate them, causing subsequent filesystem destruction.

Problem is, it's fairly easy for things like backup applications to get
stuck onthe swapfile, sleeping until someone does a swapoff.

So take all that out again and add a new S_SWAPFILE inode flag.  Test that
in the truncate path and refuse to truncate an in-use swapfile.

Synchronisation between swapon and truncate is via i_sem.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>