mm/hmm.c: remove superfluous RCU protection around radix tree lookup

mm/hmm.c: remove superfluous RCU protection around radix tree lookup

hmm_devmem_find() requires rcu_read_lock_held() but there's nothing which
actually uses the RCU protection.  The only caller is
hmm_devmem_pages_create() which already grabs the mutex and does
superfluous rcu_read_lock/unlock() around the function.

This doesn't add anything and just adds to confusion.  Remove the RCU
protection and open-code the radix tree lookup.  If this needs to become
more sophisticated in the future, let's add them back when necessary.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Tejun Heo <[email protected]>
Reviewed-by: Jérôme Glisse <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Benjamin LaHaise <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>