ecryptfs: re-order a condition for static checkers

Static checkers complain that we are using "s->i" as an offset before
we check whether it is within bounds.  It doesn't matter much but we
can easily swap the order of the checks to make everyone happy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>