ext4: make sure ext4_has_free_blocks returns 0 for ENOSPC

Fix ext4_has_free_blocks() to return 0 when we don't have enough space.

Signed-off-by: Aneesh Kumar K.V <[e-mail]>
Signed-off-by: Mingming Cao <[e-mail]>
Signed-off-by: "Theodore Ts'o" <[e-mail]>

fs/ext4/balloc.c

···
 		free_blocks =
 			percpu_counter_sum_and_set(&sbi->s_freeblocks_counter);
 #endif
+	if (free_blocks <= root_blocks)
+		/* we don't have free space */
+		return 0;
 	if (free_blocks - root_blocks < nblocks)
 		return free_blocks - root_blocks;
 	return nblocks;