One part of the system can attempt to use a mapped device before another has
finished initialising it or while it is being freed.

This patch introduces a place holder value, MINOR_ALLOCED, to mark the minor
as allocated but in a state where it can't be used, such as mid-allocation or
mid-free.  At the end of the initialization, it replaces the place holder with
the pointer to the mapped_device, making it available to the rest of the dm
subsystem.

[akpm: too late for 2.6.17 - suitable for 2.6.17.x after it has settled]
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>