btrfs-progs: check: get the highest inode for lost+found

btrfs-progs: check: get the highest inode for lost+found

root->highest_inode is not accurate at the time of creating a lost+found
and it fails because the highest_inode+1 is already present. This could be
because of fixes after highest_inode is set. Instead, search
for the highest inode in the tree and use it for lost+found.

This makes root->highest_inode unnecessary and hence deleted.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Reviewed-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.com>