net: Fix ip_mc_{dec,inc}_group allocation context

After 4effd28c ("bridge: join all-snoopers multicast address"), I
started seeing the following sleep in atomic warnings:

[   26.763893] BUG: sleeping function called from invalid context at mm/slab.h:421
[   26.771425] in_atomic(): 1, irqs_disabled(): 0, pid: 1658, name: sh
[   26.777855] INFO: lockdep is turned off.
[   26.781916] CPU: 0 PID: 1658 Comm: sh Not tainted 5.0.0-rc4 #20
[   26.787943] Hardware name: BCM97278SV (DT)
[   26.792118] Call trace:
[   26.794645]  dump_backtrace+0x0/0x170
[   26.798391]  show_stack+0x24/0x30
[   26.801787]  dump_stack+0xa4/0xe4
[   26.805182]  ___might_sleep+0x208/0x218
[   26.809102]  __might_sleep+0x78/0x88
[   26.812762]  kmem_cache_alloc_trace+0x64/0x28c
[   26.817301]  igmp_group_dropped+0x150/0x230
[   26.821573]  ip_mc_dec_group+0x1b0/0x1f8
[   26.825585]  br_ip4_multicast_leave_snoopers.isra.11+0x174/0x190
[   26.831704]  br_multicast_toggle+0x78/0xcc
[   26.835887]  store_bridge_parm+0xc4/0xfc
[   26.839894]  multicast_snooping_store+0x3c/0x4c
[   26.844517]  dev_attr_store+0x44/0x5c
[   26.848262]  sysfs_kf_write+0x50/0x68
[   26.852006]  kernfs_fop_write+0x14c/0x1b4
[   26.856102]  __vfs_write+0x60/0x190
[   26.859668]  vfs_write+0xc8/0x168
[   26.863059]  ksys_write+0x70/0xc8
[   26.866449]  __arm64_sys_write+0x24/0x30
[   26.870458]  el0_svc_common+0xa0/0x11c
[   26.874291]  el0_svc_handler+0x38/0x70
[   26.878120]  el0_svc+0x8/0xc

while toggling the bridge's multicast_snooping attribute dynamically.

Pass a gfp_t down to igmpv3_add_delrec(), introduce
__igmp_group_dropped() and introduce __ip_mc_dec_group() to take a gfp_t
argument.

Similarly introduce ____ip_mc_inc_group() and __ip_mc_inc_group() to
allow caller to specify gfp_t.

IPv6 part of the patch appears fine.

Fixes: 4effd28c ("bridge: join all-snoopers multicast address")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>