btrfs: move btrfs_find_highest_objectid/btrfs_find_free_objectid to disk-io.c

stable inclusion
from stable-v5.10.181
commit 9c69a9d05824521e2e3a615283e8158772a1a5f7
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8GJZJ

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=9c69a9d05824521e2e3a615283e8158772a1a5f7



--------------------------------

[ Upstream commit ec7d6dfd ]

Those functions are going to be used even after inode cache is removed
so moved them to a more appropriate place.

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: 0004ff15 ("btrfs: fix space cache inconsistency after error loading it from disk")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>