pidfs: move O_RDWR into pidfs_alloc_file()

JIRA: https://issues.redhat.com/browse/RHEL-107520
Conflicts:
  A merge conflict due to missing upstream commit a71f402 ("pidfs:
  get rid of __pidfd_prepare()") and related ones.

commit c57f07b
Author: Christian Brauner <brauner@kernel.org>
Date:   Mon, 14 Apr 2025 15:55:05 +0200

    pidfs: move O_RDWR into pidfs_alloc_file()

    Since all pidfds must be O_RDWR currently enfore that directly in the
    file allocation function itself instead of letting callers specify it.

    Link: https://lore.kernel.org/20250414-work-coredump-v2-1-685bf231f828@kernel.org
    Tested-by: Luca Boccassi <luca.boccassi@gmail.com>
    Reviewed-by: Oleg Nesterov <oleg@redhat.com>
    Signed-off-by: Christian Brauner <brauner@kernel.org>

Signed-off-by: Waiman Long <longman@redhat.com>

Author: Waiman-Long

fs/pidfs.c

@@ -708,6 +708,7 @@ struct file *pidfs_alloc_file(struct pid *pid, unsigned int flags)
 	if (ret < 0)
 		return ERR_PTR(ret);
 
+	flags |= O_RDWR;
 	pidfd_file = dentry_open(&path, flags, current_cred());
 	path_put(&path);
 	return pidfd_file;