smb3: increase initial number of credits requested to allow write

Compared to other clients the Linux smb3 client ramps up
credits very slowly, taking more than 128 operations before a
maximum size write could be sent (since the number of credits
requested is only 2 per small operation, causing the credit
limit to grow very slowly).

This lack of credits initially would impact large i/o performance,
when large i/o is tried early before enough credits are built up.

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