nilfs2: add FITRIM ioctl support for nilfs2

Add support for the FITRIM ioctl, which enables user space tools to
issue TRIM/DISCARD requests to the underlying device.  Every clean
segment within the specified range will be discarded.

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>