s390/pkey: Rework EP11 pkey handler to use stack for small memory allocs

There have been some places in the EP11 handler code where relatively
small amounts of memory have been allocated an freed at the end
of the function. This code has been reworked to use the stack instead.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/20250424133619.16495-21-freude@linux.ibm.com


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