eCryptfs: fix Tag 11 parsing code

Fix up the Tag 11 parsing code to handle size limits and boundaries more
explicitly.  Pay attention to *8* bytes for the key identifier (literal data),
no more, no less.

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>