Patch from Manfred Spraul

f_version and i_version are used by filesystems to check if it can
reuse the f_pos position across readdir calls without validation.

Right now f_version and i_version are modified by
    f_version = ++event;
    i_version = ++event;
    if (f_version != i_version) goto revalidate
and event is a global, exported variable.

But that's not needed,
    f_version  = 0;
    i_version++;
    if (f_version != i_version) goto revalidate
works too, without the ugly 'event' variable.

I got an ok from viro, and I had notified the fs maintainers, no
complaints either

- block_dev.c, block_llseek updates f_version to '++event'.
   grep showed that no device driver uses f_version, this is dead
   code copied from the default llseek implementation.

- the llseek implementations and get_empty_flip set f_version
   to '++event'
   This is not dead code, but
            filp->f_version = 0
   achieves the same effect:
   f_version is used by the readdir() implementation of several
   filesystems to skip the revalidation of f_pos at the beginning
   of a readdir call: If llseek was not called and the filesystem
   did not change since the last readdir call, then the value in
   f_pos can be trusted.
   The implementation (for example in ext2) is
     inode->i_version = ++event;
   in all operations that change a directory
   At the beginning of file_operation->readdir():
     if(inode->i_version != flip->f_version)
                revalidate();
     filp->f_version = inode->i_version;
   There are other users of f_version, but none of them use the
   default llseek implementation (e.g. fs/pipe.c)