xfs: catch invalid negative blknos in _xfs_buf_find()

Here blkno is a daddr_t, which is a __s64; it's possible to hold
a value which is negative, and thus pass the (blkno >= eofs)
test.  Then we try to do a xfs_perag_get() for a ridiculous
agno via xfs_daddr_to_agno(), and bad things happen when that
fails, and returns a null pag which is dereferenced shortly
thereafter.

Found via a user-supplied fuzzed image...
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NMark Tinguely <tinguely@sgi.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>

fs/xfs/xfs_buf.c

@@ -461,7 +461,7 @@ _xfs_buf_find(
 	 * have to check that the buffer falls within the filesystem bounds.
 	 */
 	eofs = XFS_FSB_TO_BB(btp->bt_mount, btp->bt_mount->m_sb.sb_dblocks);
-	if (blkno >= eofs) {
+	if (blkno < 0 || blkno >= eofs) {
 		/*
 		 * XXX (dgc): we should really be returning -EFSCORRUPTED here,
 		 * but none of the higher level infrastructure supports