anon_inode: raise SB_I_NODEV and SB_I_NOEXEC

It isn't possible to execute anonymous inodes because they cannot be
opened in any way after they have been created. This includes execution:

execveat(fd_anon_inode, "", NULL, NULL, AT_EMPTY_PATH)

Anonymous inodes have inode->f_op set to no_open_fops which sets
no_open() which returns ENXIO. That means any call to do_dentry_open()
which is the endpoint of the do_open_execat() will fail. There's no
chance to execute an anonymous inode. Unless a given subsystem overrides
it ofc.

However, we should still harden this and raise SB_I_NODEV and
SB_I_NOEXEC on the superblock itself so that no one gets any creative
ideas.

Link: https://lore.kernel.org/[email protected]
Reviewed-by: Jeff Layton <[email protected]>
Cc: [email protected] # all LTS kernels
Signed-off-by: Christian Brauner <[email protected]>

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