target: Allow userspace to write 1 to attrib/emulate_fua_write

target: Allow userspace to write 1 to attrib/emulate_fua_write

Before 4.0, reading attrib/emulate_fua_write has returned 1. Saved
configs created on a pre-4.0 kernel will try to write that back when
restoring LIO configuration. This should succeed with no effect,
and issue a warning.

See https://bugzilla.redhat.com/show_bug.cgi?id=1206184

Reported-by: Yanko Kaneti <[email protected]>
Reported-by: Juan Quintela <[email protected]>
Signed-off-by: Andy Grover <[email protected]>
Signed-off-by: Nicholas Bellinger <[email protected]>