mm/vmalloc: rework vmap_area_lock

With the new allocation approach introduced in the 5.2 kernel, it
becomes possible to get rid of one global spinlock.  By doing that we
can further improve the KVA from the performance point of view.

Basically we can have two independent locks, one for allocation part and
another one for deallocation, because of two different entities: "free
data structures" and "busy data structures".

As a result, allocation/deallocation operations can still interfere
between each other in case of running simultaneously on different CPUs,
it means there is still dependency, but with two locks it becomes lower.

Summarizing:
  - it reduces the high lock contention
  - it allows to perform operations on "free" and "busy"
    trees in parallel on different CPUs. Please note it
    does not solve scalability issue.

Test results:

In order to evaluate this patch, we can run "vmalloc test driver" to see
how many CPU cycles it takes to complete all test cases running
sequentially.  All online CPUs run it so it will cause a high lock
contention.

HiKey 960, ARM64, 8xCPUs, big.LITTLE:

<snip>
    sudo ./test_vmalloc.sh sequential_test_order=1
<snip>

<default>
[  390.950557] All test took CPU0=457126382 cycles
[  391.046690] All test took CPU1=454763452 cycles
[  391.128586] All test took CPU2=454539334 cycles
[  391.222669] All test took CPU3=455649517 cycles
[  391.313946] All test took CPU4=388272196 cycles
[  391.410425] All test took CPU5=384036264 cycles
[  391.492219] All test took CPU6=387432964 cycles
[  391.578433] All test took CPU7=387201996 cycles
<default>

<patched>
[  304.721224] All test took CPU0=391521310 cycles
[  304.821219] All test took CPU1=393533002 cycles
[  304.917120] All test took CPU2=392243032 cycles
[  305.008986] All test took CPU3=392353853 cycles
[  305.108944] All test took CPU4=297630721 cycles
[  305.196406] All test took CPU5=297548736 cycles
[  305.288602] All test took CPU6=297092392 cycles
[  305.381088] All test took CPU7=297293597 cycles
<patched>

~14%-23% patched variant is better.

Link: http://lkml.kernel.org/r/20191022155800.20468-1-urezki@gmail.com
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sonymobile.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>