s390/pkey: Use kfree_sensitive() to fix Coccinelle warnings

Replace memzero_explicit() and kfree() with kfree_sensitive() to fix
warnings reported by Coccinelle:

WARNING opportunity for kfree_sensitive/kvfree_sensitive (line 1506)
WARNING opportunity for kfree_sensitive/kvfree_sensitive (line 1643)
WARNING opportunity for kfree_sensitive/kvfree_sensitive (line 1770)

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/ZjqZkNi_JUJu73Rg@octinomon.home


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>