rxrpc: Fix irq-disabled in local_bh_enable()

rxrpc: Fix irq-disabled in local_bh_enable()

The rxrpc_assess_MTU_size() function calls down into the IP layer to find
out the MTU size for a route.  When accepting an incoming call, this is
called from rxrpc_new_incoming_call() which holds interrupts disabled
across the code that calls down to it.  Unfortunately, the IP layer uses
local_bh_enable() which, config dependent, throws a warning if IRQs are
enabled:

WARNING: CPU: 1 PID: 5544 at kernel/softirq.c:387 __local_bh_enable_ip+0x43/0xd0
...
RIP: 0010:__local_bh_enable_ip+0x43/0xd0
...
Call Trace:
 <TASK>
 rt_cache_route+0x7e/0xa0
 rt_set_nexthop.isra.0+0x3b3/0x3f0
 __mkroute_output+0x43a/0x460
 ip_route_output_key_hash+0xf7/0x140
 ip_route_output_flow+0x1b/0x90
 rxrpc_assess_MTU_size.isra.0+0x2a0/0x590
 rxrpc_new_incoming_peer+0x46/0x120
 rxrpc_alloc_incoming_call+0x1b1/0x400
 rxrpc_new_incoming_call+0x1da/0x5e0
 rxrpc_input_packet+0x827/0x900
 rxrpc_io_thread+0x403/0xb60
 kthread+0x2f7/0x310
 ret_from_fork+0x2a/0x230
 ret_from_fork_asm+0x1a/0x30
...
hardirqs last  enabled at (23): _raw_spin_unlock_irq+0x24/0x50
hardirqs last disabled at (24): _raw_read_lock_irq+0x17/0x70
softirqs last  enabled at (0): copy_process+0xc61/0x2730
softirqs last disabled at (25): rt_add_uncached_list+0x3c/0x90

Fix this by moving the call to rxrpc_assess_MTU_size() out of
rxrpc_init_peer() and further up the stack where it can be done without
interrupts disabled.

It shouldn't be a problem for rxrpc_new_incoming_call() to do it after the
locks are dropped as pmtud is going to be performed by the I/O thread - and
we're in the I/O thread at this point.

Fixes: a2ea9a907260 ("rxrpc: Use irq-disabling spinlocks between app and I/O thread")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Junvyyang, Tencent Zhuque Lab <zhuque@tencent.com>
cc: LePremierHomme <kwqcheii@proton.me>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250717074350.3767366-2-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>