cleanup truncate handling in ecryptfs

Hi Tyler,

this series cleans up the truncate handling in ecryptfs.  I did
it as preparation for some changes into size changing truncate
VFS interfaces I'm looking into in the moment.  The changes have
passed the regression test suite in the userspace ecryptfs
repository and against the ecryptfs next branch.

Diffstat:
 inode.c |  228 +++++++++++++++++++++++++++-------------------------------------
 1 file changed, 98 insertions(+), 130 deletions(-)