target_core_alua: check for buffer overflow

When a writing to a command-provided buffer we need to ensure
that we're not writing past the end of it.
At the same time we need to continue processing as typically
the final data length (ie the required size of the buffer)
need to be returned.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>