smb3: smbdirect no longer experimental

clarify Kconfig to indicate that smb direct
(SMB3 over RDMA) is no longer experimental.

Over the last three releases Long Li has
fixed various problems uncovered by xfstesting.

Signed-off-by: Steve French <stfrench@microsoft.com>
Acked-by: Ronnie Sahlberg <lsahlber@redhat.com>