btrfs: inode: cleanup the log-tree exceptions in btrfs_truncate_inode_items()

There are a lot of root owner checks in btrfs_truncate_inode_items()
like:

	if (test_bit(BTRFS_ROOT_SHAREABLE, &root->state) ||
	    root == fs_info->tree_root)

But considering that, only these trees can have INODE_ITEMs:

- tree root (for v1 space cache)
- subvolume trees
- tree reloc trees
- data reloc tree
- log trees

And since subvolume/tree reloc/data reloc trees all have SHAREABLE bit,
and we're checking tree root manually, so above check is just excluding
log trees.

This patch will replace two of such checks to a simpler one:

	if (root->root_key.objectid != BTRFS_TREE_LOG_OBJECTID)

This would merge btrfs_drop_extent_cache() and lock_extent_bits() call
into the same if branch.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>