x86/mm: Update comment in detect_tme() regarding x86_phys_bits

x86/mm: Update comment in detect_tme() regarding x86_phys_bits

As Kai pointed out, the primary reason for adjusting x86_phys_bits is to
reflect that the the address space is reduced and not the ability to
communicate the available physical address space to virtual machines.

Suggested-by: Kai Huang <[email protected]>
Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]