dm thin: allocate the cell_sort_array dynamically

commit a822c83e47d97cdef38c4352e1ef62d9f46cfe98 upstream.

Given the pool's cell_sort_array holds 8192 pointers it triggers an
order 5 allocation via kmalloc.  This order 5 allocation is prone to
failure as system memory gets more fragmented over time.

Fix this by allocating the cell_sort_array using vmalloc.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
[bwh: Backported to 3.2: make a similar change in prison_{create,destroy}()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --cc drivers/md/dm-thin.c
Simple merge