[JFFS2] Speed up mount for directly-mapped NOR flash

Remove excessive scanning of empty flash after a clean
marker for users of the point/unpoint method. cfi_cmdset_0001
uses point/unpoint by default iff flash mapping is linear.
The speedup is several orders of magnitude if FS is less than
half full.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>