net: cpsw_new: Execute ndo_set_rx_mode callback in a work queue

In the Linux kernel, the following vulnerability has been resolved:

net: cpsw_new: Execute ndo_set_rx_mode callback in a work queue

Commit 1767bb2d47b7 ("ipv6: mcast: Don't hold RTNL for
IPV6_ADD_MEMBERSHIP and MCAST_JOIN_GROUP.") removed the RTNL lock for
IPV6_ADD_MEMBERSHIP and MCAST_JOIN_GROUP operations. However, this
change triggered the following call trace on my BeagleBone Black board:
WARNING: net/8021q/vlan_core.c:236 at vlan_for_each+0x120/0x124, CPU#0: rpcbind/496
RTNL: assertion failed at net/8021q/vlan_core.c (236)
Modules linked in:
CPU: 0 UID: 997 PID: 496 Comm: rpcbind Not tainted 6.19.0-rc6-next-20260122-yocto-standard+ #8 PREEMPT
Hardware name: Generic AM33XX (Flattened Device Tree)
Call trace:
unwind_backtrace from show_stack+0x28/0x2c
show_stack from dump_stack_lvl+0x30/0x38
dump_stack_lvl from __warn+0xb8/0x11c
__warn from warn_slowpath_fmt+0x130/0x194
warn_slowpath_fmt from vlan_for_each+0x120/0x124
vlan_for_each from cpsw_add_mc_addr+0x54/0xd8
cpsw_add_mc_addr from __hw_addr_ref_sync_dev+0xc4/0xec
__hw_addr_ref_sync_dev from __dev_mc_add+0x78/0x88
__dev_mc_add from igmp6_group_added+0x84/0xec
igmp6_group_added from __ipv6_dev_mc_inc+0x1fc/0x2f0
__ipv6_dev_mc_inc from __ipv6_sock_mc_join+0x124/0x1b4
__ipv6_sock_mc_join from do_ipv6_setsockopt+0x84c/0x1168
do_ipv6_setsockopt from ipv6_setsockopt+0x88/0xc8
ipv6_setsockopt from do_sock_setsockopt+0xe8/0x19c
do_sock_setsockopt from __sys_setsockopt+0x84/0xac
__sys_setsockopt from ret_fast_syscall+0x0/0x5

This trace occurs because vlan_for_each() is called within
cpsw_ndo_set_rx_mode(), which expects the RTNL lock to be held.
Since modifying vlan_for_each() to operate without the RTNL lock is not
straightforward, and because ndo_set_rx_mode() is invoked both with and
without the RTNL lock across different code paths, simply adding
rtnl_lock() in cpsw_ndo_set_rx_mode() is not a viable solution.

To resolve this issue, we opt to execute the actual processing within
a work queue, following the approach used by the icssg-prueth driver.