Charge at least one credit, if server says that it supports multicredit

In SMB2.1 and later the server will usually set the large MTU flag, and
we need to charge at least one credit, if server says that since
it supports multicredit.  Windows seems to let us get away with putting
a zero there, but they confirmed that it is wrong and the spec says
to put one there (if the request is under 64K and the CAP_LARGE_MTU
was returned during protocol negotiation by the server.

CC: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Steve French <smfrench@gmail.com>