tree-wide: check if return value of lseek() and friends is negative

We usually check return value of syscalls or glibc functions by it is
negative or not, something like that `if (stat(path, &st) < 0)`.
Let's also use the same style for lseek() and friends even the type of
their return value is off_t.

Note, fseeko() returns int, instead of off_t.

src/machine/machined-dbus.c

@@ -613,7 +613,7 @@ static int clean_pool_done(Operation *operation, int ret, sd_bus_error *error) {
         assert(operation);
         assert(operation->extra_fd >= 0);
 
-        if (lseek(operation->extra_fd, 0, SEEK_SET) == (off_t) -1)
+        if (lseek(operation->extra_fd, 0, SEEK_SET) < 0)
                 return -errno;
 
         f = take_fdopen(&operation->extra_fd, "r");