test/mbuf: skip field registration at busy offset

test/mbuf: skip field registration at busy offset

There is a test for dynamic field registration at a specific offset.
Depending on which driver is probed, some dynamic fields may be
already registered at this offset.
This failure is skipped with a warning.

Fixes: 4958ca3a443a ("mbuf: support dynamic fields and flags")
Cc: stable@dpdk.org
Reported-by: David Marchand <david.marchand@redhat.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Marchand <david.marchand@redhat.com>