Since Linux 4.6, mtd->priv no longer points to the NAND specific
structure. Under 4.9 it contains NULL, thus using it to access
chip->options causes an invalid pointer dereference (FS#1200).
Update the code to use the mtd_to_nand() helper under 4.9 to obtain
the address of the chip specific data.
Fixes: 7bbf4117c6 ("ar71xx: Add kernel 4.9 support")
Signed-off-by: Gabor Juhos <juhosg@freemail.hu>
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