scsi_transport_iscsi: return -EOVERFLOW for Too many iscsi targets

setting err as -EOVERFLOW for Too many iscsi targets.

Also fixes a spurious compiler warning for gcc 4.3.3 and gcc 4.4 :

  CC      drivers/scsi/scsi_transport_iscsi.o
drivers/scsi/scsi_transport_iscsi.c: In function ‘iscsi_add_session’:
drivers/scsi/scsi_transport_iscsi.c:678: warning: ‘err’ may be used uninitialized in this function

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>
Acked-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>