spin off cifs_setattr with unix extensions to its own function

Create a new cifs_setattr_unix function to handle a setattr when unix
extensions are enabled and have cifs_setattr call it. Also, clean up
variable declarations in cifs_setattr.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>