pidfs: raise SB_I_NODEV and SB_I_NOEXEC

Similar to commit 1ed95281 ("anon_inode: raise SB_I_NODEV and SB_I_NOEXEC"):
it shouldn't be possible to execute pidfds via
execveat(fd_anon_inode, "", NULL, NULL, AT_EMPTY_PATH)
so raise SB_I_NOEXEC so that no one gets any creative ideas.

Also raise SB_I_NODEV as we don't expect or support any devices on pidfs.

Link: https://lore.kernel.org/20250618-work-pidfs-persistent-v2-1-98f3456fd552@kernel.org


Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>