ext4: fix a BUG in mb_mark_used during trim.

In a bs=4096 volume, if we call FITRIM with the following parameter as
fstrim_range(start = 102400, len = 134144000, minlen = 10240),
we will trigger this BUG_ON:

	BUG_ON(start + len > (e4b->bd_sb->s_blocksize << 3));

Mar  4 00:55:52 boyu-tm kernel: ------------[ cut here ]------------
Mar  4 00:55:52 boyu-tm kernel: kernel BUG at fs/ext4/mballoc.c:1506!
Mar  4 01:21:09 boyu-tm kernel: Code: d4 00 00 00 00 49 89 fe 8b 56 0c 44 8b 7e 04 89 55 c4 48 8b 4f 28 89 d6 44 01 fe 48 63 d6 48 8b 41 18 48 c1 e0 03 48 39 c2 76 04 <0f> 0b eb fe 48 8b 55 b0 8b 47 34 3b 42 08 74 04 0f 0b eb fe 48
Mar  4 01:21:09 boyu-tm kernel: RIP  [<ffffffffa053eb42>] mb_mark_used+0x47/0x26c [ext4]
Mar  4 01:21:09 boyu-tm kernel:  RSP <ffff880121e45c38>
Mar  4 01:21:09 boyu-tm kernel: ---[ end trace 9f461696f6a9dcf2 ]---

Fix this bug by doing the accounting correctly.

Cc: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>