mtd: mtdchar: retry large buffer allocations

mtd: mtdchar: retry large buffer allocations

Replace direct call to kmalloc for a potentially large, contiguous
buffer allocation with one to mtd_kmalloc_up_to which helps ensure the
operation can succeed under low-memory, highly- fragmented situations
albeit somewhat more slowly.

Signed-off-by: Grant Erickson <[email protected]>
Tested-by: Ben Gardiner <[email protected]>
Tested-by: Stefano Babic <[email protected]>
Signed-off-by: Artem Bityutskiy <[email protected]>
Signed-off-by: David Woodhouse <[email protected]>