rtslib: make write_protect=0 create read-only mapped LUNs
rtslib: make write_protect=0 create read-only mapped LUNs

The write_protect parameter was incorrectly parsed so that whenever it
was specified, even with the value 0, it set the mapped LUN read-only.

	cd /iscsi/iqn.0000-00.a.b/tpgt1/acls/iqn.0000-00.de.suse:foo
	create tpg_lun=0 mapped_lun=0 write_protect=0

Merge-Fixes-By: Andy Grover <b7d524d2f5cc5aebadb6b92b08d3ab26911cde33@redhat.com>

agrover/rtslib-fb,mikenawrocki/rtslib-fb,mikenawrocki/rtslib-fb,cvubrugier/rtslib-fb