ore: fix BUG_ON, too few sgs when reading

ore: fix BUG_ON, too few sgs when reading

commit 361aba569f55dd159b850489a3538253afbb3973 upstream.

When reading RAID5 files, in rare cases, we calculated too
few sg segments. There should be two extra for the beginning
and end partial units.

Also "too few sg segments" should not be a BUG_ON there is
all the mechanics in place to handle it, as a short read.
So just return -ENOMEM and the rest of the code will gracefully
split the IO.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>