FSAL_LUSTRE: adds fsdev to fsal_handle to avoid collision in cache_inode

This commit adds field "fsdev" to fsal_handle. This field holds the FS'devid
from stat() (field st_dev in struct stat). If various LUSTRE are mounted on
a single Ganesha, handle could collide in cache_inode hash. Such a change
avoid this.

Signed-off-by: Philippe DENIEL <philippe.deniel@cea.fr>