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>