eCryptfs: check for eCryptfs cipher support at mount

eCryptfs: check for eCryptfs cipher support at mount

commit 5f5b331d5c21228a6519dcb793fc1629646c51a6 upstream.

The issue occurs when eCryptfs is mounted with a cipher supported by
the crypto subsystem but not by eCryptfs. The mount succeeds and an
error does not occur until a write. This change checks for eCryptfs
cipher support at mount time.

Resolves Launchpad issue #338914, reported by Tyler Hicks in 03/2009.
https://bugs.launchpad.net/ecryptfs/+bug/338914

Signed-off-by: Tim Sally <tsally@atomicpeace.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>