ksmbd: decrease the number of SMB3 smbdirect server SGEs

The server-side SMBDirect layer requires no more than 6 send SGEs
The previous default of 8 causes ksmbd to fail on the SoftiWARP
(siw) provider, and possibly others. Additionally, large numbers
of SGEs reduces performance significantly on adapter implementations.

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