ext4 crypto: enforce context consistency

Enforce the following inheritance policy:

1) An unencrypted directory may contain encrypted or unencrypted files
or directories.

2) All files or directories in a directory must be protected using the
same key as their containing directory.

As a result, assuming the following setup:

mke2fs -t ext4 -Fq -O encrypt /dev/vdc
mount -t ext4 /dev/vdc /vdc
mkdir /vdc/a /vdc/b /vdc/c
echo foo | e4crypt add_key /vdc/a
echo bar | e4crypt add_key /vdc/b
for i in a b c ; do cp /etc/motd /vdc/$i/motd-$i ; done

Then we will see the following results:

cd /vdc
mv a b			# will fail; /vdc/a and /vdc/b have different keys
mv b/motd-b a		# will fail, see above
ln a/motd-a b		# will fail, see above
mv c a	    		# will fail; all inodes in an encrypted directory
   	  		#	must be encrypted
ln c/motd-c b		# will fail, see above
mv a/motd-a c		# will succeed
mv c/motd-a a		# will succeed

Signed-off-by: Michael Halcrow <mhalcrow@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>