cifs: fix static checker warning

Remove unnecessary NULL check of oparam->cifs_sb when parsing symlink
error response as it's already set by all smb2_open_file() callers and
deferenced earlier.

This fixes below report:

  fs/cifs/smb2file.c:126 smb2_open_file()
  warn: variable dereferenced before check 'oparms->cifs_sb' (see line 112)

Link: https://lore.kernel.org/r/Y0kt42j2tdpYakRu@kili


Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>