Add test for maximum SMB valid pwrite range

We need to validate we can write above 16 TiB offset and that
we get properly stopped writing about 64 TiB offset.

tests/api2/test_smb_write_offset.py

@@ -0,0 +1,60 @@
+import pytest
+import random
+
+from middlewared.test.integration.assets.account import user, group
+from middlewared.test.integration.assets.pool import dataset
+
+from protocols import smb_connection
+
+from samba import ntstatus
+from samba import NTSTATUSError
+
+SHARE_NAME = 'offset_test'
+
+LARGE_OFFSET = 0x0000140000000000
+INVALID_OFFSET = 0x0000410000000000
+
+
+@pytest.fixture(scope='module')
+def setup_smb_tests():
+    with dataset('smbclient-testing', data={'share_type': 'SMB'}) as ds:
+        with user({
+            'username': 'smbuser',
+            'full_name': 'smbuser',
+            'group_create': True,
+            'password': 'Abcd1234'
+        }) as u:
+            with smb_share(os.path.join('/mnt', ds), SHARE_NAME) as s:
+                try:
+                    call('service.start', 'cifs')
+                    yield {'dataset': ds, 'share': s, 'user': u}
+                finally:
+                    call('service.stop', 'cifs')
+
+
+def test_valid_offset(setup_smb_tests):
+    ds, share, smb_user = setup_smb_tests
+    with smb_connection(
+        share=SHARE_NAME,
+        username=smb_user['username'],
+        password='Abcd1234',
+        smb1=False
+    ) as c:
+        fd = c.create_file('file_valid_offset', 'w')
+        buf = random.randbytes(1024)
+
+        c.write(fd, offset=LARGE_OFFSET, data=buf)
+        out = c.read(fd, offset=LARGE_OFFSET, cnt=1024)
+        assert buf == out
+
+
+def test_invalid_offset(setup_smb_tests):
+    ds, share, smb_user = setup_smb_tests
+    with smb_connection(
+        share=SHARE_NAME,
+        username=smb_user['username'],
+        password='Abcd1234',
+        smb1=False
+    ) as c:
+        fd = c.create_file('file_valid_offset', 'w')
+        c.write(fd, offset=INVALID_OFFSET, data=b'CANARY')