[MTD] [NAND] Correctly validate out-of-band offset and length

Add checks to ensure that out-of-band reads and writes are
not attempted with an invalid offset or length.  Specifically,
the offset must be less than the size of oob for a page
and the length must not go beyond the size of the device.
Additionally the checks must adjust for auto-placement
(MTD_OOB_AUTO) of oob data.

Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>