block/nfs: fix potential segfault on early callback

it will happen in the future that the callback of a libnfs call
directly invokes the callback. In this case we end up in a segfault
because the NFSRPC is gone when we the BH is scheduled.

Signed-off-by: Peter Lieven <pl@kamp.de>
block/iscsi: fix potential segfault on early callback

it might happen in the future that a function directly invokes its callback.
In this case we end up in a segfault because the iTask is gone when the BH
is scheduled.

Signed-off-by: Peter Lieven <pl@kamp.de>
block/iscsi: use 16 byte CDBs also for big requests

READ10, WRITE10, WRITESAME10 only support requests up to
65536 blocks. This is 32MB for 512-Byte sectors. Change
to READ16, WRITE16, WRITESAME16 just in case such a big
request is issued.

Signed-off-by: Peter Lieven <pl@kamp.de>