CVE-2025-38032: mr: consolidate the ipmr_can_free_table() checks.

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

mr: consolidate the ipmrcanfreetable() checks.

Guoyu Yin reported a splat in the ipmr netns cleanup path:

WARNING: CPU: 2 PID: 14564 at net/ipv4/ipmr.c:440 ipmrfreetable net/ipv4/ipmr.c:440 [inline] WARNING: CPU: 2 PID: 14564 at net/ipv4/ipmr.c:440 ipmrrulesexit+0x135/0x1c0 net/ipv4/ipmr.c:361 Modules linked in: CPU: 2 UID: 0 PID: 14564 Comm: syz.4.838 Not tainted 6.14.0 #1 Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:ipmrfreetable net/ipv4/ipmr.c:440 [inline] RIP: 0010:ipmrrulesexit+0x135/0x1c0 net/ipv4/ipmr.c:361 Code: ff df 48 c1 ea 03 80 3c 02 00 75 7d 48 c7 83 60 05 00 00 00 00 00 00 5b 5d 41 5c 41 5d 41 5e e9 71 67 7f 00 e8 4c 2d 8a fd 90 <0f> 0b 90 eb 93 e8 41 2d 8a fd 0f b6 2d 80 54 ea 01 31 ff 89 ee e8 RSP: 0018:ffff888109547c58 EFLAGS: 00010293 RAX: 0000000000000000 RBX: ffff888108c12dc0 RCX: ffffffff83e09868 RDX: ffff8881022b3300 RSI: ffffffff83e098d4 RDI: 0000000000000005 RBP: ffff888104288000 R08: 0000000000000000 R09: ffffed10211825c9 R10: 0000000000000001 R11: ffff88801816c4a0 R12: 0000000000000001 R13: ffff888108c13320 R14: ffff888108c12dc0 R15: fffffbfff0b74058 FS: 00007f84f39316c0(0000) GS:ffff88811b100000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007f84f3930f98 CR3: 0000000113b56000 CR4: 0000000000350ef0 Call Trace: <TASK> ipmrnetexitbatch+0x50/0x90 net/ipv4/ipmr.c:3160 opsexitlist+0x10c/0x160 net/core/netnamespace.c:177 setupnet+0x47d/0x8e0 net/core/netnamespace.c:394 copynetns+0x25d/0x410 net/core/netnamespace.c:516 createnewnamespaces+0x3f6/0xaf0 kernel/nsproxy.c:110 unsharensproxynamespaces+0xc3/0x180 kernel/nsproxy.c:228 ksysunshare+0x78d/0x9a0 kernel/fork.c:3342 dosysunshare kernel/fork.c:3413 [inline] sesysunshare kernel/fork.c:3411 [inline] x64sysunshare+0x31/0x40 kernel/fork.c:3411 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xa6/0x1a0 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x77/0x7f RIP: 0033:0x7f84f532cc29 Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007f84f3931038 EFLAGS: 00000246 ORIGRAX: 0000000000000110 RAX: ffffffffffffffda RBX: 00007f84f5615fa0 RCX: 00007f84f532cc29 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000040000400 RBP: 00007f84f53fba18 R08: 0000000000000000 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000 R13: 0000000000000000 R14: 00007f84f5615fa0 R15: 00007fff51c5f328 </TASK>

The running kernel has CONFIGIPMROUTEMULTIPLETABLES disabled, and the sanity check for such build is still too loose.

Address the issue consolidating the relevant sanity check in a single helper regardless of the kernel configuration. Also share it between the ipv4 and ipv6 code.