cifs: add spinlock for the openFileList to cifsInodeInfo

commit 487317c9 upstream.

We can not depend on the tcon->open_file_lock here since in multiuser mode
we may have the same file/inode open via multiple different tcons.

The current code is race prone and will crash if one user deletes a file
at the same time a different user opens/create the file.

To avoid this we need to have a spinlock attached to the inode and not the tcon.

RHBZ:  1580165

Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Pavel Shilovsky <pshilov@microsoft.com>
[bwh: Backported to 3.16: adjust context, indentation]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>