LU-14712 ldiskfs: introduce EXT4_BG_TRIMMED to optimize fstrim

LU-14712 ldiskfs: introduce EXT4_BG_TRIMMED to optimize fstrim

Currently the flag indicating block group has done fstrim is not
persistent, and trim status will be lost after remount, as
a result fstrim can not skip the already trimmed groups, which
could be slow on very large devices.

This patch introduces a new block group flag EXT4_BG_TRIMMED,
we need 1 extra block group descriptor write after trimming each
block group.
When clearing the flag, the block group descriptor is journalled
already so no extra overhead.

Add a new super block flag EXT2_FLAGS_TRACK_TRIM, to indicate if
we should honour EXT4_BG_TRIMMED when doing fstrim.
The new super block flag can be turned on/off via tune2fs.

Change-Id: I7faaca4754b1726ad05d0aafe3e90e0e9f591617
Test-Parameters: trivial
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/51923
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Sergey Cheremencev <scherementsev@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>