fs: move i_readcount

On a 64-bit system, a hole exists in the 'inode' structure after
i_writecount.  This patch moves i_readcount to fill this hole.

Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: David Howells <dhowells@redhat.com>