btrfs: make btrfs_new_extent_direct take btrfs_inode

This function really needs a btrfs_inode and not a generic vfs one. Take
it as a parameter and get rid of superfluous BTRFS_I() calls.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>