FROMGIT: f2fs: add sysfs nodes to get GC info for each GC mode

Added gc_reclaimed_segments and gc_segment_mode sysfs nodes.
1) "gc_reclaimed_segments" shows how many segments have been
reclaimed by GC during a specific GC mode.
2) "gc_segment_mode" is used to control for which gc mode
the "gc_reclaimed_segments" node shows.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Bug: 182708936
(cherry picked from commit 07c6b593
 git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git

 dev)
Change-Id: Ie8c2ccf5d36ce2f388c98b77e22848f3ff6645c3
Signed-off-by: Daeho Jeong <daehojeong@google.com>