JFS: 0 is not valid errno value so return NULL from jfs_lookup

Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Signed-off-by: Dave Kleikamp <shaggy@linux.vnet.ibm.com>
Cc: jfs-discussion@lists.sourceforge.net
Cc: Alexander Viro <viro@zeniv.linux.org.uk>