lib: bpf_legacy: fix bpffs mount when /sys/fs/bpf exists

bpf selftests using iproute2 fails with:

$ ip link set dev veth0 xdp object ../bpf/xdp_dummy.o section xdp_dummy
Continuing without mounted eBPF fs. Too old kernel?
mkdir (null)/globals failed: No such file or directory
Unable to load program

This happens when the /sys/fs/bpf directory exists. In this case, mkdir
in bpf_mnt_check_target() fails with errno == EEXIST, and the function
returns -1. Thus bpf_get_work_dir() does not call bpf_mnt_fs() and the
bpffs is not mounted.

Fix this in bpf_mnt_check_target(), returning 0 when the mountpoint
exists.

Fixes: d4fcdbbec9 ("lib/bpf: Fix and simplify bpf_mnt_check_target()")
Reported-by: Mingyu Shi <mshi@redhat.com>
Reported-by: Jiri Benc <jbenc@redhat.com>
Suggested-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

lib/bpf_legacy.c

@@ -513,9 +513,12 @@ static int bpf_mnt_check_target(const char *target)
 	int ret;
 
 	ret = mkdir(target, S_IRWXU);
-	if (ret && errno != EEXIST)
+	if (ret) {
+		if (errno == EEXIST)
+			return 0;
 		fprintf(stderr, "mkdir %s failed: %s\n", target,
 			strerror(errno));
+	}
 
 	return ret;
 }