btrfs: remove extent_io_tree_to_inode() and is_inode_io_tree()

btrfs: remove extent_io_tree_to_inode() and is_inode_io_tree()

These functions aren't used outside extent-io-tree.c, but yet one of them
(extent_io_tree_to_inode()) is unnecessarily exported in the header.

Furthermore their single use is in a pattern like this:

    if (is_inode_io_tree(tree))
        foo(extent_io_tree_to_inode(tree), ...);

So we're effectively unnecessarily adding more indirection, checking
twice if tree->owner == IO_TREE_INODE_IO before getting the inode and
doing a non-inline function call to get tree->inode.

Simplify this by removing these helper functions and instead doing
thing like this:

   if (tree->owner == IO_TREE_INODE_IO)
       foo(tree->inode, ...);

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>