This patch use file system specific ioctl for getting i_generation
value. Not all file system support the ioctl. So we add an export
specific extended operation and assign right callback for the
file system that support i_generation ioctl

["M. Mohan Kumar" <mohan@in.ibm.com> we can do ioctl only for
regular files and directories on the server]
Signed-off-by: NHarsh Prateek Bora <harsh@linux.vnet.ibm.com>
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>