smb3: smbdirect support can be configured by default

smbdirect support (SMB3 over RDMA) should be enabled by
default in many configurations.

It is not experimental and is stable enough and has enough
performance benefits to recommend that it be configured by
default.  Change the  "If unsure N" to "If unsure Y" in
the description of the configuration parameter.

Acked-by: Aurelien Aptel <aaptel@suse.com>
Reviewed-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>