nilfs2: add ioctl which limits range of segment to be allocated

This adds a new ioctl command which limits range of segment to be
allocated.  This is intended to gather data whithin a range of the
partition before shrinking the filesystem, or to control new log
location for some purpose.

If a range is specified by the ioctl, segment allocator of nilfs tries
to allocate new segments from the range unless no free segments are
available there.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>