Add optional callback before each device gets suspended (called 'presuspend').
Rename existing callback used by dm-mirror from 'suspend' to 'postsuspend'.

dm-multipath will use the new callback.

(Any kernel module using device-mapper must be recompiled after this patch.)
Signed-Off-By: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>