util: Make ns_enter() a void function and report setns() errors

ns_enter() returns an integer... but it's always zero.  If we actually fail
the function doesn't return.  Therefore it makes more sense for this to be
a function returning void, and we can remove the cases where we pointlessly
checked its return value.

In addition ns_enter() is usually called from an ephemeral thread created
by NS_CALL().  That means that the exit(EXIT_FAILURE) there usually won't
be reported (since NS_CALL() doesn't wait() for the thread).  So, use die()
instead to print out some information in the unlikely event that our
setns() here does fail.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

util.h

@@ -216,7 +216,7 @@ int bitmap_isset(const uint8_t *map, int bit);
 char *line_read(char *buf, size_t len, int fd);
 void procfs_scan_listen(struct ctx *c, uint8_t proto, int ip_version, int ns,
 			uint8_t *map, uint8_t *exclude);
-int ns_enter(const struct ctx *c);
+void ns_enter(const struct ctx *c);
 bool ns_is_init(void);
 void write_pidfile(int fd, pid_t pid);
 int __daemon(int pidfile_fd, int devnull_fd);