KVM: Open code kvm_set_memory_region() into its sole caller (ioctl() API)

Open code kvm_set_memory_region() into its sole caller in preparation for
adding a dedicated API for setting internal memslots.

Oppurtunistically use the fancy new guard(mutex) to avoid a local 'r'
variable.

Cc: Tao Su <tao1.su@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Acked-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
Link: https://lore.kernel.org/r/20250111002022.1230573-2-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>