cifs: cleanups for cifs_mkdir_qinfo

Rename inode pointers for better clarity. Move the d_instantiate call to
the end of the function to prevent other tasks from seeing it before
we've finished constructing it. Since we should have exclusive access to
the inode at this point, remove the spinlock around i_nlink update.

Reviewed-by: Pavel Shilovsky <piastry@etersoft.ru>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>