The call to exportfs_encode_inode_fh() takes an pointer
as the 4th argument, so replace the integer 0 with the
NULL pointer.

This fixes the following sparse warning:

fs/notify/fdinfo.c:53:87: warning: Using plain integer as NULL pointer

Link: https://lore.kernel.org/r/20191016095955.3347-1-ben.dooks@codethink.co.ukSigned-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Jan Kara <jack@suse.cz>