ipv4: Provide a FIB flushing signal from nexthop removal functions

ipv4: Provide a FIB flushing signal from nexthop removal functions

Plumb a bool value throughout the various nexthop removal functions,
determined in the innermost __remove_nexthop_fib() (which still does the
FIB flushing) and propagated up all callers.

The next patch will make use of this signal to optimize the removal of
multiple nexthops by moving the FIB flushing up the call hierarchy.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20260507075606.322405-2-cratiu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>