[PATCH] dm: suspend: fix error path

If the device is already suspended, just return the error and skip the code
that would incorrectly wipe md->suspended_bdev.

(This isn't currently a problem because existing code avoids calling this
function if the device is already suspended.)

Signed-off-by: Alasdair G Kergon <agk@redhat.com>
Cc: <dm-devel@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>