JFFS2: Speed up and fix comparison functions

Copying complete nodes from flash can be slow if the flash is slow
to read. By only reading the data needed, the sorting operation can
be made much faster.

The directory entry comparison function also had a two bugs. First, it
did not ensure the name was copied, so the name comparison may have
been faulty (although it would have worked with NOR flash).  Second,
setting the ino to zero to ignore the entry did not work, since this
was either writing to a temporary buffer, or (for NOR flash) directly
to flash. Either way, the change was not remembered.

Signed-off-by: Mark Tomlinson <mark.tomlinson@alliedtelesis.co.nz>