eCryptfs: fix Tag 11 writing code

Fix up the Tag 11 writing code to handle size limits and boundaries more
explicitly.  It looks like the packet length was 1 shorter than it should have
been, chopping off the last byte of the key identifier.  This is largely
inconsequential, since it is not much more likely that a key identifier
collision will occur with 7 bytes rather than 8.  This patch fixes the packet
to use the full number of bytes that were originally intended to be used for
the key identifier.

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>