target: Remove enum transport_lunflags_table

se_dev_entry.lun_flags and se_lun.lun_access are only used for keeping
track of read-write vs. read-only state. Since this is an either/or thing
we can represent it as bool, and remove the unneeded enum
transport_lunflags_table, which is left over from when there were more
flags.

Change code that uses this enum to just use true/false, and make it clear
through variable and param names that true means read-only, false means
read-write.

Signed-off-by: Andy Grover <agrover@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>