[CIFS] Remove unnecessary checks

file->f_path.dentry or file->f_path.dentry.d_inode can't be NULL since at
least ten years, similar for all but very few arguments passed in from the
VFS.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>