cifs: fix lock length calculation

The lock length was wrongly set to 0 when fl_end == OFFSET_MAX, thus
failing to lock the whole file when l_start=0 and l_len=0.

This fixes test 2 from cthon04.

Before patch:

$ ./cthon04/lock/tlocklfs -t 2 /mnt

Creating parent/child synchronization pipes.

Test #1 - Test regions of an unlocked file.
        Parent: 1.1  - F_TEST  [               0,               1] PASSED.
        Parent: 1.2  - F_TEST  [               0,          ENDING] PASSED.
        Parent: 1.3  - F_TEST  [               0,7fffffffffffffff] PASSED.
        Parent: 1.4  - F_TEST  [               1,               1] PASSED.
        Parent: 1.5  - F_TEST  [               1,          ENDING] PASSED.
        Parent: 1.6  - F_TEST  [               1,7fffffffffffffff] PASSED.
        Parent: 1.7  - F_TEST  [7fffffffffffffff,               1] PASSED.
        Parent: 1.8  - F_TEST  [7fffffffffffffff,          ENDING] PASSED.
        Parent: 1.9  - F_TEST  [7fffffffffffffff,7fffffffffffffff] PASSED.

Test #2 - Try to lock the whole file.
        Parent: 2.0  - F_TLOCK [               0,          ENDING] PASSED.
        Child:  2.1  - F_TEST  [               0,               1] FAILED!
        Child:  **** Expected EACCES, returned success...
        Child:  **** Probably implementation error.

**  CHILD pass 1 results: 0/0 pass, 0/0 warn, 1/1 fail (pass/total).
        Parent: Child died

** PARENT pass 1 results: 10/10 pass, 0/0 warn, 0/0 fail (pass/total).

After patch:

$ ./cthon04/lock/tlocklfs -t 2 /mnt

Creating parent/child synchronization pipes.

Test #2 - Try to lock the whole file.
        Parent: 2.0  - F_TLOCK [               0,          ENDING] PASSED.
        Child:  2.1  - F_TEST  [               0,               1] PASSED.
        Child:  2.2  - F_TEST  [               0,          ENDING] PASSED.
        Child:  2.3  - F_TEST  [               0,7fffffffffffffff] PASSED.
        Child:  2.4  - F_TEST  [               1,               1] PASSED.
        Child:  2.5  - F_TEST  [               1,          ENDING] PASSED.
        Child:  2.6  - F_TEST  [               1,7fffffffffffffff] PASSED.
        Child:  2.7  - F_TEST  [7fffffffffffffff,               1] PASSED.
        Child:  2.8  - F_TEST  [7fffffffffffffff,          ENDING] PASSED.
        Child:  2.9  - F_TEST  [7fffffffffffffff,7fffffffffffffff] PASSED.
        Parent: 2.10 - F_ULOCK [               0,          ENDING] PASSED.

** PARENT pass 1 results: 2/2 pass, 0/0 warn, 0/0 fail (pass/total).

**  CHILD pass 1 results: 9/9 pass, 0/0 warn, 0/0 fail (pass/total).

Fixes: d80c6984

 ("cifs: fix signed integer overflow when fl_end is OFFSET_MAX")
Reported-by: Xiaoli Feng <xifeng@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>