target: Handle 0 correctly in transport_get_sectors_6()

SBC-3 says:

    A TRANSFER LENGTH field set to zero specifies that 256 logical
    blocks shall be written.  Any other value specifies the number
    of logical blocks that shall be written.

The old code was always just returning the value in the TRANSFER LENGTH
byte.  Fix this to return 256 if the byte is 0.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Cc: stable@kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_transport.c

@@ -2144,10 +2144,15 @@ static inline u32 transport_get_sectors_6(
 
 	/*
 	 * Everything else assume TYPE_DISK Sector CDB location.
-	 * Use 8-bit sector value.
+	 * Use 8-bit sector value.  SBC-3 says:
+	 *
+	 *   A TRANSFER LENGTH field set to zero specifies that 256
+	 *   logical blocks shall be written.  Any other value
+	 *   specifies the number of logical blocks that shall be
+	 *   written.
 	 */
 type_disk:
-	return (u32)cdb[4];
+	return cdb[4] ? : 256;
 }
 
 static inline u32 transport_get_sectors_10(