pnfsblock: acquire im_lock in _preload_range

commit 39e567ae upstream.

When calling _add_entry, we should take the im_lock to protect
agains other modifiers.

Signed-off-by: Peng Tao <peng_tao@emc.com>
Signed-off-by: Benny Halevy <bhalevy@tonian.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>