f2fs: sysfs: introduce max_lock_elapsed_time

f2fs: sysfs: introduce max_lock_elapsed_time

This patch add a new sysfs node in /sys/fs/f2fs/<device>/max_lock_elapsed_time.

This is a threshold, once a thread enters critical region that lock covers,
total elapsed time exceeds this threshold, f2fs will print tracepoint to dump
information of related context. This sysfs entry can be used to control the
value of threshold, by default, the value is 500 ms.

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