xfs: tiny cleanup for xfs_link

The source and target inodes are guaranteed to never be the same by the VFS,
so no need to check for that (and we would get into bad trouble later anyway
if that were the case).  Also clean up the error handling to use two gotos
instead of nested conditions.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Felix Blyakher <felixb@sgi.com>