btrfs: replace calls to btrfs_find_free_ino with btrfs_find_free_objectid

[ Upstream commit abadc1fcd72e887a8f875dabe4a07aa8c28ac8af ]

The former is going away as part of the inode map removal so switch
callers to btrfs_find_free_objectid. No functional changes since with
INODE_MAP disabled (default) find_free_objectid was called anyway.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Stable-dep-of: 0004ff15ea26 ("btrfs: fix space cache inconsistency after error loading it from disk")
Signed-off-by: Sasha Levin <sashal@kernel.org>