net: ibmveth: Refactored veth_pool_store for better maintainability

Make veth_pool_store detect requested pool changes, close device if
necessary, update pool, and reopen device.

Signed-off-by: Dave Marquardt <davemarq@linux.ibm.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250506160004.328347-1-davemarq@linux.ibm.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>