Add support for a password salt stored in the superblock

Add support for a password salt stored in the superblock

Previously, e4crypt required the user to manually specify the salt
used for their passphrase.  This was user unfriendly to say the least.
The e4crypt program can now request the salt using an ioctl, which
will automatically generate the salt if necessary, and keep it in the
ext4 superblock.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>