nbd: consistently use ssize_t

GCC (pedantically, but correctly) considers that a negative ssize_t may
become positive when casted to int.  This may cause uninitialized variable
warnings when a function returns such a negative ssize_t and is inlined.
Propagate ssize_t return types to avoid this.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/nbd.h b/nbd.h
index a8382f0..217a82d 100644
--- a/nbd.h
+++ b/nbd.h
@@ -70,8 +70,8 @@
 int nbd_receive_negotiate(int csock, const char *name, uint32_t *flags,
                           off_t *size, size_t *blocksize);
 int nbd_init(int fd, int csock, uint32_t flags, off_t size, size_t blocksize);
-int nbd_send_request(int csock, struct nbd_request *request);
-int nbd_receive_reply(int csock, struct nbd_reply *reply);
+ssize_t nbd_send_request(int csock, struct nbd_request *request);
+ssize_t nbd_receive_reply(int csock, struct nbd_reply *reply);
 int nbd_client(int fd);
 int nbd_disconnect(int fd);