RDMA/rxe: Do not call dev_mc_add/del() under a spinlock

RDMA/rxe: Do not call  dev_mc_add/del() under a spinlock

These routines were not intended to be called under a spinlock and will
throw debugging warnings:

   raw_local_irq_restore() called with IRQs enabled
   WARNING: CPU: 13 PID: 3107 at kernel/locking/irqflag-debug.c:10 warn_bogus_irq_restore+0x2f/0x50
   CPU: 13 PID: 3107 Comm: python3 Tainted: G            E     5.18.0-rc1+ #7
   Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
   RIP: 0010:warn_bogus_irq_restore+0x2f/0x50
   Call Trace:
    <TASK>
    _raw_spin_unlock_irqrestore+0x75/0x80
    rxe_attach_mcast+0x304/0x480 [rdma_rxe]
    ib_attach_mcast+0x88/0xa0 [ib_core]
    ib_uverbs_attach_mcast+0x186/0x1e0 [ib_uverbs]
    ib_uverbs_handler_UVERBS_METHOD_INVOKE_WRITE+0xcd/0x140 [ib_uverbs]
    ib_uverbs_cmd_verbs+0xdb0/0xea0 [ib_uverbs]
    ib_uverbs_ioctl+0xd2/0x160 [ib_uverbs]
    do_syscall_64+0x5c/0x80
    entry_SYSCALL_64_after_hwframe+0x44/0xae

Move them out of the spinlock, it is OK if there is some races setting up
the MC reception at the ethernet layer with rbtree lookups.

Fixes: 6090a0c4c7c6 ("RDMA/rxe: Cleanup rxe_mcast.c")
Link: https://lore.kernel.org/r/20220504202817.98247-1-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>