JFFS2: Use CLEANMARKER to reduce scanning time

If a sector has a CLEANMARKER at the beginning, it indicates that the
entire sector has been erased. Therefore, if this is found, we can skip the
entire block. This was not being done before this patch.

The code now does the same as the kernel does when encountering a
CLEANMARKER. It still checks that the next few words are FFFFFFFF, and if
so, the block is assumed to be empty, and so is skipped.

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