nilfs2: add a tracepoint for tracking stage transition of segment construction

This patch adds a tracepoint for tracking stage transition of block
collection in segment construction. With the tracepoint, we can
analysis the behavior of segment construction in depth. It would be
useful for bottleneck detection and debugging, etc.

The tracepoint is created with the standard trace API of linux (like
ext3, ext4, f2fs and btrfs). So we can analysis with existing tools
easily. Of course, more detailed analysis will be possible if we can
create nilfs specific analysis tools.

Below is an example of event dump with Brendan Gregg's perf-tools
(https://github.com/brendangregg/perf-tools). Time consumption between
each stage can be obtained.

$ sudo bin/tpoint nilfs2:nilfs2_collection_stage_transition
Tracing nilfs2:nilfs2_collection_stage_transition. Ctrl-C to end.
        segctord-14875 [003] ...1 28311.067794: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_INIT
        segctord-14875 [003] ...1 28311.068139: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_GC
        segctord-14875 [003] ...1 28311.068139: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_FILE
        segctord-14875 [003] ...1 28311.068486: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_IFILE
        segctord-14875 [003] ...1 28311.068540: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_CPFILE
        segctord-14875 [003] ...1 28311.068561: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_SUFILE
        segctord-14875 [003] ...1 28311.068565: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_DAT
        segctord-14875 [003] ...1 28311.068573: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_SR
        segctord-14875 [003] ...1 28311.068574: nilfs2_collection_stage_transition: sci = ffff8800ce6de000, stage = ST_DONE

For capturing transition correctly, this patch adds wrappers for the
member scnt of nilfs_cstage. With this change, every transition of the
stage can produce trace event in a correct manner.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>