s390/mm: Reimplement lazy ASCE handling

Reduce system call overhead time (round trip time for invoking a
non-existent system call) by 25%.

With the removal of set_fs() [1] lazy control register handling was removed
in order to keep kernel entry and exit simple. However this made system
calls slower.

With the conversion to generic entry [2] and numerous follow up changes
which simplified the entry code significantly, adding support for lazy asce
handling doesn't add much complexity to the entry code anymore.

In particular this means:

- On kernel entry the primary asce is not modified and contains the user
  asce

- Kernel accesses which require secondary-space mode (for example futex
  operations) are surrounded by enable_sacf_uaccess() and
  disable_sacf_uaccess() calls. enable_sacf_uaccess() sets the primary asce
  to kernel asce so that the sacf instruction can be used to switch to
  secondary-space mode. The primary asce is changed back to user asce with
  disable_sacf_uaccess().

The state of the control register which contains the primary asce is
reflected with a new TIF_ASCE_PRIMARY bit. This is required on context
switch so that the correct asce is restored for the scheduled in process.

In result address spaces are now setup like this:

CPU running in               | %cr1 ASCE | %cr7 ASCE | %cr13 ASCE
-----------------------------|-----------|-----------|-----------
user space                   |  user     |  user     |  kernel
kernel (no sacf)             |  user     |  user     |  kernel
kernel (during sacf uaccess) |  kernel   |  user     |  kernel
kernel (kvm guest execution) |  guest    |  user     |  kernel

In result cr1 control register content is not changed except for:
- futex system calls
- legacy s390 PCI system calls
- the kvm specific cmpxchg_user_key() uaccess helper

This leads to faster system call execution.

[1] 87d59863 ("s390/mm: remove set_fs / rework address space handling")
[2] 56e62a73 ("s390: convert to generic entry")

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