eCryptfs: Move ecryptfs docs into Documentation/filesystems/

Signed-off-by: NJosef 'Jeff' Sipek <jsipek@cs.sunysb.edu>
Acked-by: NMichael Halcrow <mhalcrow@us.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>