erofs: make filesystem exportable

Implement export operations in order to make EROFS support accessing
inodes with filehandles so that it can be exported via NFS and used
by overlayfs.

Without this patch, 'exportfs -rv' will report:
exportfs: /root/erofs_mp does not support NFS export

Also tested with unionmount-testsuite and the testcase below passes now:
./run --ov --erofs --verify hard-link

For more details about the testcase, see:
https://github.com/amir73il/unionmount-testsuite/pull/6

Signed-off-by: Hongnan Li <hongnan.li@linux.alibaba.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220425040712.91685-1-hongnan.li@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>