s3:smb2_server: always try to grant the credits the client just consumed

s3:smb2_server: always try to grant the credits the client just consumed

It turns out that the effective credits_requested is always at least 1,
even if the client sends credits_requested == 0.

This means the client is not able to reduce the amount of credits
itself.

Without this fix a client (e.g. Windows7) would reach the case
where it has been granted all credits it asked for.
When copying a large file with a lot of parallel requests,
all these requests have credits_requested == 0.
This means the amount of granted credits where reduced by each
request and only when the granted credits reached 0,
the server granted one credit to allow the client to go on.
The client might require more than one credit ([MS-SMB2] says
Windows clients require at least 4 credits) and freezes
with just 1 credit.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=9702

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>