Btrfs: leave spinning on lookup and map the leaf

On lookup we only want to read the inode item, so leave the path spinning.  Also
we're just wholesale reading the leaf off, so map the leaf so we don't do a
bunch of kmap/kunmaps.  Thanks,
Signed-off-by: NJosef Bacik <josef@redhat.com>

fs/btrfs/inode.c

@@ -2493,6 +2493,7 @@ static void btrfs_read_locked_inode(struct inode *inode)
 
 	path = btrfs_alloc_path();
 	BUG_ON(!path);
+	path->leave_spinning = 1;
 	memcpy(&location, &BTRFS_I(inode)->location, sizeof(location));
 
 	ret = btrfs_lookup_inode(NULL, root, path, &location, 0);
@@ -2502,6 +2503,12 @@ static void btrfs_read_locked_inode(struct inode *inode)
 	leaf = path->nodes[0];
 	inode_item = btrfs_item_ptr(leaf, path->slots[0],
 				    struct btrfs_inode_item);
+	if (!leaf->map_token)
+		map_private_extent_buffer(leaf, (unsigned long)inode_item,
+					  sizeof(struct btrfs_inode_item),
+					  &leaf->map_token, &leaf->kaddr,
+					  &leaf->map_start, &leaf->map_len,
+					  KM_USER1);
 
 	inode->i_mode = btrfs_inode_mode(leaf, inode_item);
 	inode->i_nlink = btrfs_inode_nlink(leaf, inode_item);
@@ -2539,6 +2546,11 @@ static void btrfs_read_locked_inode(struct inode *inode)
 	if (!maybe_acls)
 		cache_no_acl(inode);
 
+	if (leaf->map_token) {
+		unmap_extent_buffer(leaf, leaf->map_token, KM_USER1);
+		leaf->map_token = NULL;
+	}
+
 	btrfs_free_path(path);
 	inode_item = NULL;