In brd_init func, rd_nr num of brd_device are firstly allocated
and add in brd_devices, then brd_devices are traversed to add each
brd_device by calling add_disk func. When allocating brd_device,
the disk->first_minor is set to i * max_part, if rd_nr * max_part
is larger than MINORMASK, two different brd_device may have the same
devt, then only one of them can be successfully added.
when rmmod brd.ko, it will cause oops when calling brd_exit.

Follow those steps:
  # modprobe brd rd_nr=3 rd_size=102400 max_part=1048576
  # rmmod brd
then, the oops will appear.

Oops log:
[  726.613722] Call trace:
[  726.614175]  kernfs_find_ns+0x24/0x130
[  726.614852]  kernfs_find_and_get_ns+0x44/0x68
[  726.615749]  sysfs_remove_group+0x38/0xb0
[  726.616520]  blk_trace_remove_sysfs+0x1c/0x28
[  726.617320]  blk_unregister_queue+0x98/0x100
[  726.618105]  del_gendisk+0x144/0x2b8
[  726.618759]  brd_exit+0x68/0x560 [brd]
[  726.619501]  __arm64_sys_delete_module+0x19c/0x2a0
[  726.620384]  el0_svc_common+0x78/0x130
[  726.621057]  el0_svc_handler+0x38/0x78
[  726.621738]  el0_svc+0x8/0xc
[  726.622259] Code: aa0203f6 aa0103f7 aa1e03e0 d503201f (7940e260)

Here, we add brd_check_and_reset_par func to check and limit max_part par.

--
V5->V6:
 - remove useless code

V4->V5:(suggested by Ming Lei)
 - make sure max_part is not larger than DISK_MAX_PARTS

V3->V4:(suggested by Ming Lei)
 - remove useless change
 - add one limit of max_part

V2->V3: (suggested by Ming Lei)
 - clear .minors when running out of consecutive minor space in brd_alloc
 - remove limit of rd_nr

V1->V2:
 - add more checks in brd_check_par_valid as suggested by Ming Lei.
Signed-off-by: NZhiqiang Liu <liuzhiqiang26@huawei.com>
Reviewed-by: NBob Liu <bob.liu@oracle.com>
Reviewed-by: NMing Lei <ming.lei@redhat.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>