dm crypt: use cond_resched

Add cond_resched() to prevent monopolising CPU when processing large bios.

dm-crypt processes encryption of bios in sector units.  If the bio request
is big it can spend a long time in the encryption call.
Signed-off-by: NMilan Broz <mbroz@redhat.com>
Tested-by: NYan Li <elliot.li.tech@gmail.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>

drivers/md/dm-crypt.c

@@ -432,6 +432,7 @@ static int crypt_convert(struct crypt_config *cc,
 		case 0:
 			atomic_dec(&ctx->pending);
 			ctx->sector++;
+			cond_resched();
 			continue;
 
 		/* error */