ksmbd: fix Out-of-Bounds Read in ksmbd_vfs_stream_read

An offset from client could be a negative value, It could lead
to an out-of-bounds read from the stream_buf.
Note that this issue is coming when setting
'vfs objects = streams_xattr parameter' in ksmbd.conf.

Cc: stable@vger.kernel.org # v5.15+
Reported-by: Jordy Zomer <jordyzomer@google.com>
Signed-off-by: Jordy Zomer <jordyzomer@google.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>