intel-iommu: Kill superfluous mapping_lock

Since we're using cmpxchg64() anyway (because that's the only way to do
an atomic 64-bit store on i386), we might as well ditch the extra
locking and just use cmpxchg64() to ensure that we don't add the page
twice.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>