Merge patch series "pidfs: preserve thread pidfds reopened by file handle"

Li Chen <me@linux.beauty> says:

Reopening a thread pidfd via open_by_handle_at() silently dropped
PIDFD_THREAD because do_dentry_open() strips O_EXCL. Restore the flag
and add a selftest check.

* patches from https://patch.msgid.link/20260716052726.1032092-1-me@linux.beauty:
  selftests/pidfd: check PIDFD_THREAD survives open_by_handle_at()
  pidfs: preserve thread pidfds reopened by file handle

Link: https://patch.msgid.link/20260716052726.1032092-1-me@linux.beauty
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>