Add buffer validation for SMB2_CREATE_CONTEXT.

Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Reviewed-by: NRalph Boehme <slow@samba.org>
Signed-off-by: NHyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: NNamjae Jeon <linkinjeon@kernel.org>
Signed-off-by: NSteve French <stfrench@microsoft.com>