samples, bpf: fix to change the buffer size for read()

BugLink: https://bugs.launchpad.net/bugs/1838467

[ Upstream commit f7c2d64b ]

If the trace for read is larger than 4096, the return
value sz will be 4096. This results in off-by-one error
on buf:

    static char buf[4096];
    ssize_t sz;

    sz = read(trace_fd, buf, sizeof(buf));
    if (sz > 0) {
        buf[sz] = 0;
        puts(buf);
    }
Signed-off-by: Chang-Hsien Tsai <luke.tw@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

samples/bpf/bpf_load.c

@@ -336,7 +336,7 @@ void read_trace_pipe(void)
 		static char buf[4096];
 		ssize_t sz;
 
-		sz = read(trace_fd, buf, sizeof(buf));
+		sz = read(trace_fd, buf, sizeof(buf) - 1);
 		if (sz > 0) {
 			buf[sz] = 0;
 			puts(buf);