eCryptfs: set inode key only once per crypto operation

There is no need to keep re-setting the same key for any given eCryptfs inode.
This patch optimizes the use of the crypto API and helps performance a bit.

Signed-off-by: Trevor Highland <trevor.highland@gmail.com>
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>