ext4: do not use yield()

Using yield() is strongly discouraged (see sched/core.c) especially
since we can just use cond_resched().

Replace all use of yield() with cond_resched().
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/inode.c

@@ -1352,7 +1352,7 @@ static int ext4_da_reserve_space(struct inode *inode, ext4_lblk_t lblock)
 		ei->i_da_metadata_calc_last_lblock = save_last_lblock;
 		spin_unlock(&ei->i_block_reservation_lock);
 		if (ext4_should_retry_alloc(inode->i_sb, &retries)) {
-			yield();
+			cond_resched();
 			goto repeat;
 		}
 		dquot_release_reservation_block(inode, EXT4_C2B(sbi, 1));

fs/ext4/mballoc.c

@@ -3692,11 +3692,7 @@ ext4_mb_discard_group_preallocations(struct super_block *sb,
 	if (free < needed && busy) {
 		busy = 0;
 		ext4_unlock_group(sb, group);
-		/*
-		 * Yield the CPU here so that we don't get soft lockup
-		 * in non preempt case.
-		 */
-		yield();
+		cond_resched();
 		goto repeat;
 	}
 
@@ -4246,7 +4242,7 @@ ext4_fsblk_t ext4_mb_new_blocks(handle_t *handle,
 			ext4_claim_free_clusters(sbi, ar->len, ar->flags)) {
 
 			/* let others to free the space */
-			yield();
+			cond_resched();
 			ar->len = ar->len >> 1;
 		}
 		if (!ar->len) {