btrfs: checking NULL or not in some functions

Because NULL is returned when the memory allocation fails,
it is checked whether it is NULL.

Signed-off-by: Tsutomu Itoh <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/extent-tree.c

···
 	int ret = 0;
 
 	ra = kzalloc(sizeof(*ra), GFP_NOFS);
+	if (!ra)
+		return -ENOMEM;
 
 	mutex_lock(&inode->i_mutex);
 	first_index = start >> PAGE_CACHE_SHIFT;

fs/btrfs/extent_io.c

···
 		nr = bio_get_nr_vecs(bdev);
 
 	bio = btrfs_bio_alloc(bdev, sector, nr, GFP_NOFS | __GFP_HIGH);
+	if (!bio)
+		return -ENOMEM;
 
 	bio_add_page(bio, page, page_size, offset);
 	bio->bi_end_io = end_io_func;

fs/btrfs/tree-log.c

···
 	log = root->log_root;
 
 	path = btrfs_alloc_path();
+	if (!path)
+		return -ENOMEM;
 	dst_path = btrfs_alloc_path();
+	if (!dst_path) {
+		btrfs_free_path(path);
+		return -ENOMEM;
+	}
 
 	min_key.objectid = inode->i_ino;
 	min_key.type = BTRFS_INODE_ITEM_KEY;