s4:torture: add tests to test the SMB2 read/write offset/length boundaries

s4:torture: add tests to test the SMB2 read/write offset/length boundaries

[MS-FSA] 2.1.5.2 Server Requests a Read and
2.1.5.3 Server Requests a Write define some contraints.

These tests demonstrate that ((int64_t)offset) < 0) is
not allowed for both reads and writes for SMB.
Also the special case for writes at offset -2 is not possible
nor the append mode with offset < 0.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14361

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>