cifs: use type __u32 instead of int for the oplock parameter

... and avoid implicit casting from a signed type. Also, pass oplock by value
instead by reference as we don't intend to change the value in
cifs_open_inode_helper().

Thanks to Jeff Layton for spotting this.

Reviewed-by: Jeff Layton <jlayton@samba.org>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>