f2fs: fix lock priority inversion issue

f2fs: fix lock priority inversion issue

If userspace thread has held f2fs rw semaphore, due to its low priority,
it could be runnable or preempted state for long time, during the time,
it will block high priority thread which is trying to grab the same rw
semaphore, e.g. cp_rwsem, io_rwsem...

To fix such issue, let's detect thread's priority when it tries to grab
f2fs_rwsem lock, if the priority is lower than a priority threshold, let's
uplift the priority before it enters into critical region of lock, and
restore the priority after it leaves from critical region.

Meanwhile, introducing two new sysfs nodes:
- /sys/fs/f2fs/<disk>/adjust_lock_priority, it is used to control whether
the functionality is enable or not.
==========     ==================
Flag_Value     Flag_Description
==========     ==================
0x00000000     Disabled (default)
0x00000001     cp_rwsem
0x00000002     node_change
0x00000004     node_write
0x00000008     gc_lock
0x00000010     cp_global
0x00000020     io_rwsem
==========     ==================
- /sys/fs/f2fs/<disk>/lock_duration_priority, it is used to control
priority threshold.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>