ext3: Adjust trim start with first_data_block.

As we have make the consense in the e-mail[1], the trim start should
be added with first_data_block. So this patch fulfill it and remove
the check for start < first_data_block.

[1] http://www.spinics.net/lists/linux-ext4/msg22737.html



Cc: Jan Kara <jack@suse.cz>
Cc: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Jan Kara <jack@suse.cz>