pidfs: add pidfs_dentry_open() helper

pidfs: add pidfs_dentry_open() helper

Both pidfs_alloc_file() and pidfs_export_open() need to force O_RDWR
and reapply the pidfd flags that do_dentry_open() strips. Move the
common logic into a helper.

PIDFD_AUTOKILL is now part of the restore mask in the file handle path
as well, but pidfs_export_permission() rejects O_TRUNC, so this is a
no-op there. But warn nonetheless.

Link: https://patch.msgid.link/20260722-esszimmer-umsetzen-nennt-ed5fc604300a@brauner
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>