ksmbd: remove smb2_buf_length in smb2_hdr

[ Upstream commit cb451720 ]

To move smb2_hdr to smbfs_common, This patch remove smb2_buf_length
variable in smb2_hdr. Also, declare smb2_get_msg function to get smb2
request/response from ->request/response_buf.

Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>