[SCSI] ibmvfc: Fix rport add/delete race resulting in oops

Commit 43c8da907ccc656935d1085701f4db83385d8a59 introduced a race
condition which can occur when adding/deleting rports. There are
two possible threads now that can be deleting rports in the ibmvfc
driver, which can result in list_del being called twice, resulting
in an oops. This patch adds a new state to the ibmvfc_target struct
to indicate the target has been removed from the list and is in
the process of being deleted.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>