s390/uaccess: provide inline variants of get_user/put_user

This shortens the code by ~17k (performace_defconfig, march=z196).
The number of exception table entries however increases from 164
entries to 2500 entries (+~18k).
However the executed code is shorter and also faster since we save
the branches to the out-of-line copy_to/from_user implementations.

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