btrfs: do not check -EAGAIN when truncating inodes in the log root

We only throttle the btrfs_truncate_inode_items if the root is
SHAREABLE, which isn't set on the log root, which means this loop is
unnecessary.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>