Overview:
Attempt to allocate a new mlx5 MR entry when global Btree cache is full ends up
calling mlx5_malloc with the EXTERNAL_HEAP_MIN_SOCKET_ID socket ID, given that
no external-memory heap has been created. Instead the rte_extmem_* API is used.

Steps to reproduce:
- start a primary DPDK process, on a NIC compatible with mlx5_core driver;
- use rte_extmem_register() to register >512 pages of 4KB;
- use rte_dev_dma_map() to dma-map each page;
- rte_eth_tx_burst() an mbuf with an external buffer from the last page of the
registered memory (or a page above index 512). (a virtual address that will not
be found in the global Btree cache);

Actual results:
mlx5_malloc in mlx5_mr_create_primary() fails with "Unable to allocate memory
for a new MR". From this point forward, packets never reach the other end.

Expected results:
MR entries should be successfully retrieved from backup or created when cache
becomes full; calling mlx5_malloc() on external heap socket should not be
possible when rte_extmem_* API is used. As it is stated in the DPDK
documentation[1], "Memory added this way will not be available for any regular
DPDK allocators".

Build Date & Hardware:
20 Jun 2023 on Debian GNU/Linux 4.18.0

[1]: https://doc.dpdk.org/guides-21.11/prog_guide/env_abstraction_layer.html