vdpa/mlx5: Fix release of uninitialized resources on error path

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

vdpa/mlx5: Fix release of uninitialized resources on error path

The commit in the fixes tag made sure that mlx5_vdpa_free()
is the single entrypoint for removing the vdpa device resources
added in mlx5_vdpa_dev_add(), even in the cleanup path of
mlx5_vdpa_dev_add().

This means that all functions from mlx5_vdpa_free() should be able to
handle uninitialized resources. This was not the case though:
mlx5_vdpa_destroy_mr_resources() and mlx5_cmd_cleanup_async_ctx()
were not able to do so. This caused the splat below when adding
a vdpa device without a MAC address.

This patch fixes these remaining issues:

- Makes mlx5_vdpa_destroy_mr_resources() return early if called on
uninitialized resources.

- Moves mlx5_cmd_init_async_ctx() early on during device addition
because it can't fail. This means that mlx5_cmd_cleanup_async_ctx()
also can't fail. To mirror this, move the call site of
mlx5_cmd_cleanup_async_ctx() in mlx5_vdpa_free().

An additional comment was added in mlx5_vdpa_free() to document
the expectations of functions called from this context.

Splat:

mlx5_core 0000:b5:03.2: mlx5_vdpa_dev_add:3950:(pid 2306) warning: No mac address provisioned?
------------[ cut here ]------------
WARNING: CPU: 13 PID: 2306 at kernel/workqueue.c:4207 __flush_work+0x9a/0xb0
[...]
Call Trace:
<TASK>
? __try_to_del_timer_sync+0x61/0x90
? __timer_delete_sync+0x2b/0x40
mlx5_vdpa_destroy_mr_resources+0x1c/0x40 [mlx5_vdpa]
mlx5_vdpa_free+0x45/0x160 [mlx5_vdpa]
vdpa_release_dev+0x1e/0x50 [vdpa]
device_release+0x31/0x90
kobject_cleanup+0x37/0x130
mlx5_vdpa_dev_add+0x327/0x890 [mlx5_vdpa]
vdpa_nl_cmd_dev_add_set_doit+0x2c1/0x4d0 [vdpa]
genl_family_rcv_msg_doit+0xd8/0x130
genl_family_rcv_msg+0x14b/0x220
? __pfx_vdpa_nl_cmd_dev_add_set_doit+0x10/0x10 [vdpa]
genl_rcv_msg+0x47/0xa0
? __pfx_genl_rcv_msg+0x10/0x10
netlink_rcv_skb+0x53/0x100
genl_rcv+0x24/0x40
netlink_unicast+0x27b/0x3b0
netlink_sendmsg+0x1f7/0x430
__sys_sendto+0x1fa/0x210
? ___pte_offset_map+0x17/0x160
? next_uptodate_folio+0x85/0x2b0
? percpu_counter_add_batch+0x51/0x90
? filemap_map_pages+0x515/0x660
__x64_sys_sendto+0x20/0x30
do_syscall_64+0x7b/0x2c0
? do_read_fault+0x108/0x220
? do_pte_missing+0x14a/0x3e0
? __handle_mm_fault+0x321/0x730
? count_memcg_events+0x13f/0x180
? handle_mm_fault+0x1fb/0x2d0
? do_user_addr_fault+0x20c/0x700
? syscall_exit_work+0x104/0x140
entry_SYSCALL_64_after_hwframe+0x76/0x7e
RIP: 0033:0x7f0c25b0feca
[...]
---[ end trace 0000000000000000 ]---