ksmbd: handle error cases first in smb2_create_sd_buffers

For code cleanup, handle error cases first in
smb2_create_sd_buffers().

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Steve French <stfrench@microsoft.com>