drm/vkms: Fix memory leak in vkms_init()

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

drm/vkms: Fix memory leak in vkms_init()

A memory leak was reported after the vkms module install failed.

unreferenced object 0xffff88810bc28520 (size 16):
comm "modprobe", pid 9662, jiffies 4298009455 (age 42.590s)
hex dump (first 16 bytes):
01 01 00 64 81 88 ff ff 00 00 dc 0a 81 88 ff ff ...d............
backtrace:
[<00000000e7561ff8>] kmalloc_trace+0x27/0x60
[<000000000b1954a0>] 0xffffffffc45200a9
[<00000000abbf1da0>] do_one_initcall+0xd0/0x4f0
[<000000001505ee87>] do_init_module+0x1a4/0x680
[<00000000958079ad>] load_module+0x6249/0x7110
[<00000000117e4696>] __do_sys_finit_module+0x140/0x200
[<00000000f74b12d2>] do_syscall_64+0x35/0x80
[<000000008fc6fcde>] entry_SYSCALL_64_after_hwframe+0x46/0xb0

The reason is that the vkms_init() returns without checking the return
value of vkms_create(), and if the vkms_create() failed, the config
allocated at the beginning of vkms_init() is leaked.

vkms_init()
config = kmalloc(...) # config allocated
...
return vkms_create() # vkms_create failed and config is leaked

Fix this problem by checking return value of vkms_create() and free the
config if error happened.