eCryptfs: kmem_cache objects for multiple keys; init/exit functions

Introduce kmem_cache objects for handling multiple keys per inode.  Add calls
in the module init and exit code to call the key list
initialization/destruction functions.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>