ext4: mark group as trimmed only if it was fully scanned

Otherwise nonaligned fstrim calls will works inconveniently for iterative
scanners, for example:

// trim [0,16MB] for group-1, but mark full group as trimmed
fstrim  -o $((1024*1024*128)) -l $((1024*1024*16)) ./m
// handle [16MB,16MB] for group-1, do nothing because group already has the flag.
fstrim  -o $((1024*1024*144)) -l $((1024*1024*16)) ./m

[ Update function documentation for ext4_trim_all_free -- TYT ]

Signed-off-by: Dmitry Monakhov <dmtrmonakhov@yandex-team.ru>
Link: https://lore.kernel.org/r/1650214995-860245-1-git-send-email-dmtrmonakhov@yandex-team.ru


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org