Since Jan removed the lock_kernel()s in inode_add_bytes() and
inode_sub_bytes(), these functions have been racy.

One problematic workload has been discovered in which concurrent writepage
and truncate on SMP quickly causes i_blocks to go negative.  writepage() does
not take i_sem, and it seems that for ext2, there are no other locks in
force when inode_add_bytes() is called.

Putting the BKL back in there is not acceptable.  To fix this race I have
added a new spinlock "i_lock" to the inode.

That lock is presently used to protect i_bytes and i_blocks.  We could use it
to protect i_size as well.

The splitting of the used disk space into i_blocks and i_bytes is silly - we
should nuke all that and just have a bare loff_t i_usedbytes.   Later.