sbc: READ6/10/12/16: Add handling of residual overflow/underflow.

Version 2:
Simplify the patch based on Alexanders comments.

This patch adds support to return overflow/underflow residuals
for the cases where the transport layer (iSCSI, ...) specifies
an EDTL that is not matching the SCSI transdfer length.

Additionally, in case of residual underflow it will also clamp the size of
returned DATA-IN to the amount of data that was actually read and not the
EDTL that was requested.

This is from RFC 3720, 10.4.1 :
     bit 6 - (U) set for Residual Underflow.  In this case, the Residual
     Count indicates the number of bytes that were not transferred out
     of the number of bytes that were expected to be transferred.  For
     a bidirectional operation, the Residual Count contains the
     residual for the write operation.

I have added a specific test for this condition to the testsute in
libiscsi :

    ./bin/iscsi-test-cu -t "TestRead10.testRead10Residuals" \
        iscsi://127.0.0.1/iqn.ronnie.test/1 -V

    Suite: TestRead10
      Test: testRead10Residuals ...
        Test invalid READ10 commands
        Block size is 512
        Try reading one block but with iSCSI expected transfer length==0
        Verify that the target returned SUCCESS
        Verify residual overflow flag is set
        Verify we got 512 bytes of residual overflow
        Try reading one block but with iSCSI expected transfer length==10000
        Verify that the target returned SUCCESS
        Verify we got one whole block back from the target
        Verify residual underflow flag is set
        Verify we got 9488 bytes of residual underflow
        Try reading one block but with iSCSI expected transfer length==200
        Verify that the target returned SUCCESS
        Verify we got 200 bytes back from the target
        Verify residual overflow flag is set
        Verify we got 312 bytes of residual overflow
        Try reading two blocks but iSCSI expected transfer length==512 (==one bl
        Verify that the target returned SUCCESS
        Verify we got one whole block back from the target
        Verify residual overflow flag is set
        Verify we got one block of residual overflow
    passed

    --Run Summary: Type      Total     Ran  Passed  Failed
                   suites        1       1     n/a       0
                   tests         1       1       1       0
                   asserts      24      24      24       0

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>