ceph: ignore error from invalidate_inode_pages2_range() in direct write

This call can fail if there are dirty pages.  The preceding call to
filemap_write_and_wait_range() will normally remove dirty pages, but
as inode_lock() is not held over calls to ceph_direct_read_write(), it
could race with non-direct writes and pages could be dirtied
immediately after filemap_write_and_wait_range() returns

If there are dirty pages, they will be removed by the subsequent call
to truncate_inode_pages_range(), so having them here is not a problem.

If the 'ret' value is left holding an error, then in the async IO case
(aio_req is not NULL) the loop that would normally call
ceph_osdc_start_request() will see the error in 'ret' and abort all
requests.  This doesn't seem like correct behaviour.

So use separate 'ret2' instead of overloading 'ret'.

Signed-off-by: NeilBrown <neilb@suse.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>