smb: client: validate minimum PDU size before smb2_get_data_area_len()

__smb2_calc_size() calls smb2_get_data_area_len(), which reads
command-specific struct fields to locate the data area. However,
smb2_check_message() only validates StructureSize2, meaning a truncated
response could cause smb2_get_data_area_len() to read out-of-bounds.

Replace has_smb2_data_area[] with smb2_min_pdu_len[], which is now
used to indicate both whether a command's response has a data area
and the size of that fixed response struct.  A non-zero entry means
the command has a data area, and is the minimum length required
before the struct is read.

For each command with a data area, PDUs shorter than this minimum size
are rejected instead of parsed.

The minimum is not applied to SMB2 error responses, which carry only
the 9-byte error body, the same exemption the StructureSize2 check
above it already makes.  STATUS_MORE_PROCESSING_REQUIRED is
treated as a normal reply, since an in-progress SESSION_SETUP
response carries a full body and a security blob.

Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>