btrfs: remove unused parameters from btrfs_lookup_inode_extref()

The function btrfs_lookup_inode_extref(` no longer requires transaction
handle, insert length, or COW flag, as the only caller now performs a
read-only lookup using trans == NULL, ins_len == 0 and cow == 0.

This function was introduced in the early days where extref feature was
introduced by commit f186373f ("btrfs: extended inode refs").
Then some cleanup was done in commit 33b98f22 ("btrfs: cleanup:
removed unused 'btrfs_get_inode_ref_index'"), which removed the only
caller passing trans and other COW specific options.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Sun YangKai <sunk67188@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>