um, x86: Cast to (u64 *) inside set_64bit()

After tightening up the types passed to set_64bit(), the cast to
(phys_t *) triggers a warning apparently because phys_t is defined as
"unsigned long" when building on 64 bits; however, u64 is defined as
"unsigned long long".  This is, however, a explicit cast inside a
size-specific call, so just make the cast explicitly (u64 *).

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Jeff Dike <jdike@addtoit.com>
LKML-Reference: <tip-69309a05@git.kernel.org>