KVM: MMU: Fix and clean up for_each_gfn_* macros

The expression (sp)->gfn should not be expanded using @gfn.
Although no user of these macros passes a string other than gfn now,
this should be fixed before anyone sees strange errors.

Note: ignored the following checkpatch errors:
  ERROR: Macros with complex values should be enclosed in parenthesis
  ERROR: trailing statements should be on next line

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>