xfs: embedd mru_elem into parent structure

There is no need to do a separate allocation for each mru element, just
embedd the structure into the parent one in the user.  Besides saving
a memory allocation and the infrastructure required for it this also
simplifies the API.

While we do major surgery on xfs_mru_cache.c also de-typedef it and
make struct mru_cache private to the implementation file.

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