UBIFS: allow for gaps when dirtying the LPT

The LPT may have gaps in it because initially empty LEBs
are not added by mkfs.ubifs - because it does not know how
many there are.  Then UBIFS allocates empty LEBs in the
reverse order that they are discovered i.e. they are
added to, and removed from, the front of a list.  That
creates a gap in the middle of the LPT.

The function dirtying the LPT tree (for the purpose of
small model garbage collection) assumed that a gap could
only occur at the very end of the LPT and stopped dirtying
prematurely, which in turn resulted in the LPT running
out of space - something that is designed to be impossible.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>