JFFS2: Add parameter to reserve disk space for root

JFFS2: Add parameter to reserve disk space for root

Add a new rp_size= parameter which creates a "reserved pool" of disk
space which can only be used by root. Other users are not permitted
to write to disk when the available space is less than the pool size.

Based on original code by Artem Bityutskiy in
https://dev.laptop.org/ticket/5317

Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@linux.intel.com>