tmpfiles: do not fail --clean when a file disappears during cleanup

dir_cleanup() stores the result of xstatx_full() in r, which is also the
function's return value. If a file is removed between readdir() and
statx(), it is skipped, but when it was the last entry r stays -ENOENT
and --clean fails with EX_CANTCREAT without logging anything. A
successful statx() also overwrote errors recorded for earlier entries.

Use a separate variable for the statx() result.

Reproducer:
  d=$(mktemp -d)
  echo "d $d - - - 1d" >$d.conf
  while :; do touch $d/f{1..200}; rm -f $d/f*; done &
  for i in $(seq 200); do systemd-tmpfiles --clean $d.conf; echo $?; done | sort | uniq -c
  kill %1

Before:
    177 0
     23 73

After:
    200 0

Follow-up for a98a6eb95c.
Follow-up for 10072d8be4.

(cherry picked from commit 4c250e3f1325f91cdee047f7daeaf532abe4af6f)

src/tmpfiles/tmpfiles.c

@@ -715,18 +715,18 @@ static int dir_cleanup(
                         continue;
 
                 struct statx sx;
-                r = xstatx_full(dirfd(d), de->d_name,
-                                AT_SYMLINK_NOFOLLOW|AT_NO_AUTOMOUNT,
-                                /* xstatx_flags= */ 0,
-                                STATX_TYPE|STATX_MODE|STATX_UID,
-                                STATX_ATIME|STATX_MTIME|STATX_CTIME|STATX_BTIME,
-                                STATX_ATTR_MOUNT_ROOT,
-                                &sx);
-                if (r == -ENOENT)
+                int k = xstatx_full(dirfd(d), de->d_name,
+                                    AT_SYMLINK_NOFOLLOW|AT_NO_AUTOMOUNT,
+                                    /* xstatx_flags= */ 0,
+                                    STATX_TYPE|STATX_MODE|STATX_UID,
+                                    STATX_ATIME|STATX_MTIME|STATX_CTIME|STATX_BTIME,
+                                    STATX_ATTR_MOUNT_ROOT,
+                                    &sx);
+                if (k == -ENOENT)
                         continue;
-                if (r < 0) {
+                if (k < 0) {
                         /* FUSE, NFS mounts, SELinux might return EACCES */
-                        log_full_errno(r == -EACCES ? LOG_DEBUG : LOG_ERR, r,
+                        log_full_errno(k == -EACCES ? LOG_DEBUG : LOG_ERR, k,
                                        "statx(%s/%s) failed: %m", p, de->d_name);
                         continue;
                 }