KVM: rename is_writeble_pte() to is_writable_pte()

There are two spellings of "writable" in
arch/x86/kvm/mmu.c and paging_tmpl.h .

This patch renames is_writeble_pte() to is_writable_pte()
and makes grepping easy.

  New name is consistent with the definition of itself:
  return pte & PT_WRITABLE_MASK;

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>