exofs: export_operations

implement export_operations and set in superblock.
It is now posible to export exofs via nfs

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>

diff --cc fs/exofs/dir.c
Simple merge

diff --cc fs/exofs/exofs.h
Simple merge

diff --cc fs/exofs/super.c
Simple merge