pidfd: cleanup the usage of __pidfd_prepare's flags

pidfd: cleanup the usage of __pidfd_prepare's flags

- make pidfd_create() static.

- Don't pass O_RDWR | O_CLOEXEC to __pidfd_prepare() in copy_process(),
  __pidfd_prepare() adds these flags unconditionally.

- Kill the flags check in __pidfd_prepare(). sys_pidfd_open() checks the
  flags itself, all other users of pidfd_prepare() pass flags = 0.

  If we need a sanity check for those other in kernel users then
  WARN_ON_ONCE(flags & ~PIDFD_NONBLOCK) makes more sense.

- Don't pass O_RDWR to get_unused_fd_flags(), it ignores everything except
  O_CLOEXEC.

- Don't pass O_CLOEXEC to anon_inode_getfile(), it ignores everything except
  O_ACCMODE | O_NONBLOCK.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20240125161734.GA778@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>