minor logdev bugs

[Andrew Tridgell <tridge@valinux.com>]

XFS doesn't open the logdev during use, which has two unfortunate
consequences.

The first is if you mount a XFS filesystem while the logdev device is
not loaded (say for example if the logdev is on a device implemented
in a kernel module). You get an oops from within linvfs_read_super()
when it calls into the pagebug to ask for pages on a device that
doesn't exist.

The second is that the usage count of the logdev module remains at
zero while the logdev is being used, which means it can be
removed. Then you get an almost certain lockup when XFS next tries to
use the log. It also means that someone else can come along and use
the devive without any usage count tests kicking in.

You can reproduce the effect by using a logdev on a ramdisk.

Cheers, Tridge