afs: Remove redundant assignment to ret

Variable ret is set to -ENOENT and -ENOMEM but this value is never
read as it is overwritten or not used later on, hence it is a
redundant assignment and can be removed.

Cleans up the following clang-analyzer warning:

fs/afs/dir.c:2014:4: warning: Value stored to 'ret' is never read
[clang-analyzer-deadcode.DeadStores].

fs/afs/dir.c:659:2: warning: Value stored to 'ret' is never read
[clang-analyzer-deadcode.DeadStores].

[DH made the following modifications:

 - In afs_rename(), -ENOMEM should be placed in op->error instead of ret,
   rather than the assignment being removed entirely.  afs_put_operation()
   will pick it up from there and return it.

 - If afs_sillyrename() fails, its error code should be placed in op->error
   rather than in ret also.
]

Fixes: e49c7b2f ("afs: Build an abstraction around an "operation" concept")
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: D...