If O_DIRECT writes are racing with buffered writes, then
the call to invalidate_inode_pages2_range() can call ceph_releasepage()
on dirty pages.

Most filesystems hold inode_lock() across O_DIRECT writes so they do not
suffer this race, but cephfs deliberately drops the lock, and opens a window
for the race.

This race can be triggered with the generic/036 test from the xfstests
test suite.  It doesn't happen every time, but it does happen often.

As the possibilty is expected, remove the warning, and instead include
the PageDirty() status in the debug message.
Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>