ocfs2: Add ocfs2_trim_fs for SSD trim support.

Add ocfs2_trim_fs to support trimming freed clusters in the
volume. A range will be given and all the freed clusters greater
than minlen will be discarded to the block layer.

Signed-off-by: Tao Ma <boyu.mt@taobao.com>
Signed-off-by: Joel Becker <jlbec@evilplan.org>