The bug which Anton found.  On the
find_or_create_page->__find_lock_page path we're performing
a read_unlock of an rwlock which is held for writing.

The patch converts that to using a write_lock throughout.

Which penalises find_lock_page() a bit.  If it shows up
on profiles then we can clone __find_lock_page() and
use read_lock()s, but for now I'd opt for saving the
cache footprint.