dm snapshot: fix metadata writing when suspending

When suspending a device-mapper device, dm_suspend() sleeps until all
necessary I/O is completed.  This state is triggered by a callback from
persistent_commit().  But some I/O can still be issued *after* the callback
(to prepare the next metadata area for use if the current one is full).  This
patch delays the callback until after that I/O is complete.

Signed-off-by: Mark McLoughlin <markmc@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>