JFFS2: 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 <marathon96@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>