LU-405 Don't assert on multiple disconnected dentries case

LU-405 Don't assert on multiple disconnected dentries case

NFS has a valid case for multiple disconnected dentries on the same
inode so we must not assert there as a workaround.
The proper fix is to adopt using of d_splice_alias eventually.

Change-Id: I28f7385d95523adf949a4e3b8887d79cdc4f2e37
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: http://review.whamcloud.com/923
Tested-by: Hudson
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>