LU-14712 e2fsprogs: support EXT2_FLAG_BG_TRIMMED and EXT2_FLAGS_TRACK_TRIM

LU-14712 e2fsprogs: support EXT2_FLAG_BG_TRIMMED and EXT2_FLAGS_TRACK_TRIM

This adds EXT2_FLAG_BG_TRIMMED, which is used on block group
descriptors during mke2fs after discard is done.
The EXT2_FLAG_BG_TRIMMED flag is cleared on the block group when
we free blocks.

Introduce EXT2_FLAGS_TRACK_TRIM, which is a new super block flag,
to indicate whether we should honour the EXT2_FLAG_BG_TRIMMED
set on each block group.
EXT2_FLAGS_TRACK_TRIM itself can be turned on/off via tune2fs.

Make dumpe2fs aware of the new flags.

Change-Id: I5af88fe4a32c8c9ab5604b84362a7140ae2cdde5
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/tools/e2fsprogs/+/51924
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>