The code EXT4_IOC_SWAP_BOOT ioctl hasn't been updated in a while, and
it's a bit broken with respect to more modern ext4 kernels, especially
metadata checksums.
Other problems fixed with this commit:
* Don't allow installing a DAX, swap file, or an encrypted file as a
boot loader.
* Respect the immutable and append-only flags.
* Wait until any DIO operations are finished *before* calling
truncate_inode_pages().
* Don't swap inode->i_flags, since these flags have nothing to do with
the inode blocks --- and it will give the IMA/audit code heartburn
when the inode is evicted.