Store atime safely when dirty_inode is called.

Store atime safely when dirty_inode is called.

When dirty_inode is called the atime might have been the only thing
updated.
If it was, then we want to record it in the atime file and not mark
the inode dirty.
If it wasn't (or if the inode is easily marked dirty) we want to
simply mark the inode dirty and make sure the correct atime is
recorded when the inode is flushed.

So detect the cases based on whether the inode dblock is available and
pinned.  When it isn't just update the atime-delta in the atime file.

Signed-off-by: NeilBrown <neilb@suse.de>