xfs: free the item in xfs_mru_cache_insert on failure

Call the provided free_func when xfs_mru_cache_insert as that's what
the callers need to do anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Hans Holmberg <hans.holmberg@wdc.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>