mshv: Fix use-after-free in mshv_map_user_memory error path

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

mshv: Fix use-after-free in mshv_map_user_memory error path

In the error path of mshv_map_user_memory(), calling vfree() directly on
the region leaves the MMU notifier registered. When userspace later unmaps
the memory, the notifier fires and accesses the freed region, causing a
use-after-free and potential kernel panic.

Replace vfree() with mshv_partition_put() to properly unregister
the MMU notifier before freeing the region.