xfs: handle duplicate entries in xfs_mru_cache_insert

The radix tree code can detect and reject duplicate keys at insert
time.  Make xfs_mru_cache_insert handle this case so that future
changes to the filestream allocator can take advantage of this.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>