Allocate memory using GFP_NOIO when deleting a btree. dm_btree_del()
can be called via an ioctl and we don't want to recurse into the FS or
block layer.
Signed-off-by: Joe Thornber <ejt@redhat.com> Signed-off-by: Mike Snitzer <snitzer@redhat.com> Signed-off-by: Ben Hutchings <ben@decadent.org.uk>