mtd: nftl: fix offset alignments

mtd: nftl: fix offset alignments

Arithmetic conversion in the mask computation makes the upper word
of the second argument passed down to mtd->read_oob(), be always 0
(assuming 'offs' being a 64-bit signed long long type, and
'mtd->writesize' being a 32-bit unsigned int type).

This patch applies over the other one adding masking in nftl_write,
"nftl: write support is broken".

Signed-off-by: Dimitri Gorokhovik <[email protected]>
Cc: Tim Gardner <[email protected]>
Cc: Scott James Remnant <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>