ksm: get_ksm_page locked

In some places where get_ksm_page() is used, we need the page to be locked.

When KSM migration is fully enabled, we shall want that to make sure that
the page just acquired cannot be migrated beneath us (raised page count is
only effective when there is serialization to make sure migration
notices).  Whereas when navigating through the stable tree, we certainly
do not want to lock each node (raised page count is enough to guarantee
the memcmps, even if page is migrated to another node).

Since we're about to add another use case, add the locked argument to
get_ksm_page() now.

Hmm, what's that rcu_read_lock() about?  Complete misunderstanding, I
really got the wrong end of the stick on that!  There's a configuration in
which page_cache_get_speculative() can do something cheaper than
get_page_unless_zero(), relying on its caller's rcu_read_lock() to have
disabled preemption for it.  There's no need for rcu_read_lock() around
get_page_unless_zero() (and mapping checks) here.  Cut out that silliness
before making this any harder to understand.

Signed-off-by: Hugh Dickins <hughd@google.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Petr Holasek <pholasek@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Izik Eidus <izik.eidus@ravellosystems.com>
Cc: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>