NAND/davinci: Fix segfault for hwecc4_infix reads

Page reads using hwecc4_infix layout segfaulted for check_bad_blocks because
the read assumed a valid data buffer, which check_bad_blocks does not use
(it only passes a 6 byte buffer for the start of OOB).

This version copes with undersized or missing data or oob buffers and uses
random read commands within the page to skip unwanted areas of data/OOB for
speed.

NOTE: Running check_bad_blocks with this layout will be reading infix
OOB locations, not manufacturer bad block markers. This means that if you
check blocks written in infix layout they will appear good, but manufacturer-
marked bad blocks may also appear good.
If you want to scan for manufactuer-marked bad blocks, you need to enable
raw_access before running check_bad_blocks, or use the non-infix layout.

Signed-off-by: Jon Povey <jon.povey@racelogic.co.uk>
CC: David Brownell <dbrownell@users.sourceforge.net>