KVM: SEV: Use PFN_DOWN() to simplify "number of pages" math when pinning memory

KVM: SEV: Use PFN_DOWN() to simplify "number of pages" math when pinning memory

Use PFN_DOWN() instead of open coded equivalents in sev_pin_memory() to
simplify the code and make it easier to read.

No functional change intended (verified before and after versions of the
generated code are identical).

Reviewed-by: Liam Merwick <liam.merwick@oracle.com>
Tested-by: Liam Merwick <liam.merwick@oracle.com>
Link: https://patch.msgid.link/20260313003302.3136111-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>