[SCSI] zfcp: Use memdup_user and kstrdup

Use the functions memdup_user and kstrdup to allocate memory and copy
the data in one step, saving some lines of code.

Reviewed-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>