fs-util: move link_fd() from tmpfile-util.c into generic fs-util.c

It's a generically useful call, let's move it so that we can use it at
more places.

src/basic/tmpfile-util.c

@@ -330,26 +330,6 @@ int fopen_tmpfile_linkable(const char *target, int flags, char **ret_path, FILE
         return 0;
 }
 
-static int link_fd(int fd, int newdirfd, const char *newpath) {
-        int r;
-
-        assert(fd >= 0);
-        assert(newdirfd >= 0 || newdirfd == AT_FDCWD);
-        assert(newpath);
-
-        /* Try symlinking via /proc/fd/ first. */
-        r = RET_NERRNO(linkat(AT_FDCWD, FORMAT_PROC_FD_PATH(fd), newdirfd, newpath, AT_SYMLINK_FOLLOW));
-        if (r != -ENOENT)
-                return r;
-
-        /* Fall back to symlinking via AT_EMPTY_PATH as fallback (this requires CAP_DAC_READ_SEARCH and a
-         * more recent kernel, but does not require /proc/ mounted) */
-        if (proc_mounted() != 0)
-                return r;
-
-        return RET_NERRNO(linkat(fd, "", newdirfd, newpath, AT_EMPTY_PATH));
-}
-
 int link_tmpfile_at(int fd, int dir_fd, const char *path, const char *target, LinkTmpfileFlags flags) {
         _cleanup_free_ char *tmp = NULL;
         int r;