ksmbd: reduce smb direct max read/write size

ksmbd does not support more than one Buffer Descriptor V1 element in
an smbdirect protocol request. Reducing the maximum read/write size to
about 512KB allows interoperability with Windows over a wider variety
of RDMA NICs, as an interim workaround.

Reviewed-by: Tom Talpey <tom@talpey.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>