mtd: nand: omap: fix ecclayout->oobfree->offset

1) In current implementation, ecclayout->oobfree->offset is calculated with
 respect to ecclayout->eccpos[0] which is incorrect because ECC bytes may not
 be stored contiguously in OOB.
 So, this patch calculates ecclayout->oobfree->offset with respect to last
 ECC byte-position 'eccpos[ecclayout->eccbytes-1]'.

2) ECC layout of some ecc-schemes expects reserved-markers at specific eccpos[]
 which should not be over-written by any file-system metadata.
 So this patch aligns oobfree->offset taking into account of such markers.

CC: <stable@vger.kernel.org> # 3.13.x+
Tested-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Tested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>