cifs: Use correct packet length in SMB2_TRANSFORM header

In smb3_init_transform_rq(), 'orig_len' was only counting the request
length, but forgot to count any data pages in the request.

Writing or creating files with the 'seal' mount option was broken.

In addition, do some code refactoring by exporting smb2_rqst_len() to
calculate the appropriate packet size and avoid duplicating the same
calculation all over the code.

The start of the io vector is either the rfc1002 length (4 bytes) or a
SMB2 header which is always > 4. Use this fact to check and skip the
rfc1002 length if requested.

Signed-off-by: Paulo Alcantara <palcantara@suse.de>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>