ksmbd: fix wrong allocation size update in smb2_open()

When client send SMB2_CREATE_ALLOCATION_SIZE create context, ksmbd update
old size to ->AllocationSize in smb2 create response. ksmbd_vfs_getattr()
should be called after it to get updated stat result.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>