selftests/bpf: fix signedness bug in redir_partial()

When xsend() returns -1 (error), the check 'n < sizeof(buf)' incorrectly
treats it as success due to unsigned promotion. Explicitly check for -1
first.

Fixes: a4b7193d ("selftests/bpf: Add sockmap test for redirecting partial skb data")
Signed-off-by: Fushuai Wang <wangfushuai@baidu.com>
Link: https://lore.kernel.org/r/20250612084208.27722-1-wangfushuai@baidu.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>