GFS2: check NULL return value in gfs2_ok_to_move

gfs2_lookupi() can return NULL if the path to the root is broken by
another rename/rmdir. In this case gfs2_ok_to_move() must check for
this NULL pointer and return error.

Resolves: rhbz#1060246
Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>