ext4: fix trimming of a single group

When ext4_trim_fs() is called to trim a part of a single group, the
logic will wrongly set last block of the interval to 'len' instead
of 'first_block + len'. Thus a shorter interval is possibly trimmed.
Fix it.

CC: Lukas Czerner <lczerner@redhat.com>
Cc: stable@kernel.org
Signed-off-by: NJan Kara <jack@suse.cz>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.c

@@ -4860,7 +4860,7 @@ int ext4_trim_fs(struct super_block *sb, struct fstrim_range *range)
 		if (len >= EXT4_BLOCKS_PER_GROUP(sb))
 			len -= (EXT4_BLOCKS_PER_GROUP(sb) - first_block);
 		else
-			last_block = len;
+			last_block = first_block + len;
 
 		if (e4b.bd_info->bb_free >= minlen) {
 			cnt = ext4_trim_all_free(sb, &e4b, first_block,