ReplicatedPG: include pending_attrs when reseting attrs in WRITEFULL

This is necessary for ops like:

2014-03-25 11:48:20.354168 7f0df0ff9700  1 -- 192.168.106.226:0/1016231
--> 192.168.106.226:6814/13647 -- osd_op(client.4198.0:153
default.4198.1_foo [create 0~0,delete,setxattr user.rgw.idtag
(15),writefull 0~3,setxattr user.rgw.manifest (407),setxattr
user.rgw.acl (133),setxattr user.rgw.content_type (25),setxattr
user.rgw.etag (33)] 3.ee3a09ea ondisk+write e49) v4 -- ?+0 0xd09830 con
0xcd74d0

Fixes: #7837
Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #1558 from ceph/wip-7837

ReplicatedPG: include pending_attrs when reseting attrs in WRITEFULL

Reviewed-by: Sage Weil <sage@inktank.com>