s390/pkey: Build module name array selectively based on kernel config options

There is a static array of pkey handler kernel module names
used in case the pkey_handler_request_modules() is invoked.
This static array is walked through and if the module is not
already loaded a module_request() is performed.

This patch reworks the code to instead of unconditionally
building up a list of module names into the array, only the
pkey handler modules available based on the current kernel
config options are inserted.

Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>