configure: actually fix Alpine mktemp issue

Apparently busybox is _really_ picky, and string must really end
in XXXXXX. So do that, and generate the .c and .o from that.

Fixes: a46158b ("Make mktemp happy on Alpine Linux")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

configure

@@ -478,8 +478,9 @@ elif command -v /usr/sbin/bpftool >/dev/null 2>&1; then
   has_bpftool="yes"
 fi
 
-tmp_bpf_c=$(mktemp /tmp/test_bpf_XXXXXX.c)
-tmp_bpf_o=$(mktemp /tmp/test_bpf_XXXXXX.o)
+tmp_bpf=$(mktemp /tmp/test_bpf_XXXXXX)
+tmp_bpf_c="$tmp_bpf".c
+tmp_bpf_o="$tmp_bpf".o
 
 cat > "$tmp_bpf_c" << EOF
 int tmp = 0;