f2fs crypto: delete an unnecessary 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 the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Samuel Pascua <pascua.samuel.14@gmail.com>