eCryptfs: Delete a check before the function call "key_put"

The key_put() function tests whether its argument is NULL and then
returns immediately. Thus the test around this call might not be needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>