fs: orangefs: fix error return code of orangefs_revalidate_lookup()

When op_alloc() returns NULL to new_op, no error return code of
orangefs_revalidate_lookup() is assigned.
To fix this bug, ret is assigned with -ENOMEM in this case.

Fixes: 8bb8aefd

 ("OrangeFS: Change almost all instances of the string PVFS2 to OrangeFS.")
Reported-by: TOTE Robot <oslab@tsinghua.edu.cn>
Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Mike Marshall <hubcap@omnibond.com>