target: fix regression with dev_link_magic in target_fabric_port_link

This is to fix a regression that only affect the stable (not for the mainline)
that the stable commit fdf9d86 was incorrectly placed dev->dev_link_magic check
before the *dev assignment in target_fabric_port_link() due to fuzzy automatically
context adjustment during the back-porting.

Reported-by: Chris Boot <bootc@bootc.net>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: CAI Qian <caiqian@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>