ceph: page still marked private_2

ceph: page still marked private_2

Previous patch that allowed us to cleanup most of the issues with pages marked
as private_2 when calling ceph_readpages. However, there seams to be a case in
the error case clean up in start read that still trigers this from time to
time. I've only seen this one a couple times.

BUG: Bad page state in process petabucket  pfn:335b82
page:ffffea000cd6e080 count:0 mapcount:0 mapping:          (null) index:0x0
page flags: 0x200000000001000(private_2)
Call Trace:
 [<ffffffff81563442>] dump_stack+0x46/0x58
 [<ffffffff8112c7f7>] bad_page+0xc7/0x120
 [<ffffffff8112cd9e>] free_pages_prepare+0x10e/0x120
 [<ffffffff8112e580>] free_hot_cold_page+0x40/0x160
 [<ffffffff81132427>] __put_single_page+0x27/0x30
 [<ffffffff81132d95>] put_page+0x25/0x40
 [<ffffffffa02cb409>] ceph_readpages+0x2e9/0x6f0 [ceph]
 [<ffffffff811313cf>] __do_page_cache_readahead+0x1af/0x260

Signed-off-by: Milosz Tanski <milosz@adfin.com>
Signed-off-by: Sage Weil <sage@inktank.com>