dpaa2-switch: rework FDB management on the bridge leave path

On bridge leave, the dpaa2_switch_port_set_fdb() function always
allocates a new FDB for the port which is becoming standalone. In case
no FDB is found, then the port leaving a bridge will continue to use the
current one.

The above logic does not cover the case in which there are multiple
bridges which have ports from the same DPSW instance. In this case, when
the last port leaves bridge #1, it finds an unused FDB to switch to, but
the old FDB is not marked as unused. Since the number of FDBs is equal
to the number of DPSW interfaces, this will eventually lead to multiple
ports sharing the same FDB.

Fix this by changing how we are managing the FDBs on the leave path.
Instead of directly allocating a new FDB, first verify if the current
port is the last one to leave a bridge. If this is the case, then
continue to use the current FDB and only allocate another FDB if there
are other ports remaining in the bridge.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://patch.msgid.link/20260528173452.1953102-2-ioana.ciornei@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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