fusefs: slightly better debugging in the tests

If the kernel rejects a response written by the server, print it. That
would most likely be due to an error in the test logic.

MFC after: 2 weeks
Sponsored by: ConnectWise

(cherry picked from commit 564c732b5c0221373d067c9a21da09b310e676ae)

fusefs: fix the 32-bit build after 564c732b5c0

It's a printf format specifier again.

MFC with: 564c732b5c0221373d067c9a21da09b310e676ae
Sponsored by: ConnectWise

(cherry picked from commit 52f7eb31ae84b0a02cd79c857c3201cb7b65c00f)