SHOOK returns: this commits repairs FSAL_LUSTRE (broken in 2.0)

The FSAL_SHOOK was broken since new_api was introduced in 2.0.
This commit repairs this feature by managing "shook restore"
from the FSAL itself.
FSAL_SHOOK still derives from FSAL_LUSTRE.
An option in Cmake (with a cmakedfine in src/include/config.h) is
used to enable/disable this feature.

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>
9P: fix export refcounting

- attach had a failure path which could keep an extra ref
- xattrwalk needs to drop the "from attach" attribute, which would cause
unwanted put_export calls.

Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>