eCryptfs: make open, truncate, and setattr use persistent file

Rather than open a new lower file for every eCryptfs file that is opened,
truncated, or setattr'd, instead use the existing lower persistent file for
the eCryptfs inode.  Change truncate to use read_write.c functions.  Change
ecryptfs_getxattr() to use the common ecryptfs_getxattr_lower() function.

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>