nilfs2: implementation of NILFS_IOCTL_SET_SUINFO ioctl

With this ioctl the segment usage entries in the SUFILE can be updated
from userspace.

This is useful, because it allows the userspace GC to modify and update
segment usage entries for specific segments, which enables it to avoid
unnecessary write operations.

If a segment needs to be cleaned, but there is no or very little
reclaimable space in it, the cleaning operation basically degrades to a
useless moving operation.  In the end the only thing that changes is the
location of the data and a timestamp in the segment usage information.
With this ioctl the GC can skip the cleaning and update the segment
usage entries directly instead.

This is basically a shortcut to cleaning the segment.  It is still
necessary to read the segment summary information, but the writing of
the live blocks can be skipped if it's not worth it.

[konishi.ryusuke@lab.ntt.co.jp: add description of NILFS_IOCTL_SET_SUINFO ioctl]
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>