cifs: Make add_credits_and_wake_if() clear deducted credits

Make add_credits_and_wake_if() clear the amount of credits in the
cifs_credits struct after it has returned them to the overall counter.
This allows add_credits_and_wake_if() to be called multiple times during
the error handling and cleanup without accidentally returning the credits
again and again.

Note that the wake_up() in add_credits_and_wake_if() may also be
superfluous as ->add_credits() also does a wake on the request_q.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Steve French <sfrench@samba.org>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Rohith Surabattula <rohiths.msft@gmail.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: linux-cifs@vger.kernel.org
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org