rbd: return errors for mapped but deleted snapshot

rbd: return errors for mapped but deleted snapshot

When a snapshot is deleted, the OSD will return ENOENT when reading
from it. This is normally interpreted as a hole by rbd, which will
return zeroes. To minimize the time in which this can happen, stop
requests early when we are notified that our snapshot no longer
exists.

[elder@inktank.com: updated __rbd_init_snaps_header() logic]

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>
(cherry picked from commit e88a36ec961b8c1899c59c5e4ae35a318c0209d3)
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Conflicts:

	drivers/block/rbd.c