scsi: target: fix SendTargets=All string compares

strncmp is currently used for "SendTargets" key and "All" value matching
without checking for trailing garbage. This means that Text request PDUs
with garbage such as "SendTargetsPlease=All" and "SendTargets=Alle" are
processed successfully as if they were "SendTargets=All" requests.

Reviewed-by: Mike Christie <mchristi@redhat.com>
Signed-off-by: David Disseldorp <ddiss@suse.de>
Link: https://lore.kernel.org/r/20190912095547.22427-3-ddiss@suse.de
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>