s390/mm: change default addressing mode

Change the default addressing mode so that user space runs in primary space
and the kernel runs in home space.
In addition remove the "switch_amode" kernel parameter so all users who
already specified they want the new default behaviour will stay in the
"switched" mode instead of in the opposite they intended.
If there is a need to switch addressing modes, this can be done with the
"user_mode" kernel parameter: user_mode=home

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>