target: Fix alua_access_state attribute OOPs for un-configured devices

target: Fix alua_access_state attribute OOPs for un-configured devices

This patch fixes a OOPs where an attempt to write to the per-device
alua_access_state configfs attribute at:

  /sys/kernel/config/target/core/$HBA/$DEV/alua/$TG_PT_GP/alua_access_state

results in an NULL pointer dereference when the backend device has not
yet been configured.

This patch adds an explicit check for DF_CONFIGURED, and fails with
-ENODEV to avoid this case.

Reported-by: Chris Boot <[email protected]>
Reported-by: Philip Gaw <[email protected]>
Cc: Chris Boot <[email protected]>
Cc: Philip Gaw <[email protected]>
Cc: [email protected] # 3.8+
Signed-off-by: Nicholas Bellinger <[email protected]>