eCryptfs: Filename Encryption: Encoding and encryption functions

These functions support encrypting and encoding the filename contents.
The encrypted filename contents may consist of any ASCII characters.  This
patch includes a custom encoding mechanism to map the ASCII characters to
a reduced character set that is appropriate for filenames.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Cc: Dustin Kirkland <dustin.kirkland@gmail.com>
Cc: Eric Sandeen <sandeen@redhat.com>
Cc: Tyler Hicks <tchicks@us.ibm.com>
Cc: David Kleikamp <shaggy@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>