eCryptfs: fix Tag 1 parsing code

Fix up the Tag 1 parsing code to handle size limits and boundaries more
explicitly.  Initialize the new auth_tok's flags.

Signed-off-by: Michael Halcrow <mhalcrow@us.ibm.com>
Cc: Josef Sipek <jsipek@fsl.cs.sunysb.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>