ecryptfs: Annotate struct ecryptfs_message with __counted_by

Add the __counted_by() compiler attribute to the flexible array member
'data' to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Acked-by: Tyler Hicks <code@tyhicks.com>
Signed-off-by: Tyler Hicks <code@tyhicks.com>