fix readahead calculations in xfs_dir2_leaf_getdents()

This is for bug #850,
http://oss.sgi.com/bugzilla/show_bug.cgi?id=850
XFS file system segfaults , repeatedly and 100% reproducable in 2.6.30 , 2.6.31

The above only showed up on a CONFIG_XFS_DEBUG=y kernel, because
xfs_bmapi() ASSERTs that it has been asked for at least one map,

and it was getting 0.

The root cause is that our guesstimated "bufsize" from xfs_file_readdir
was fairly small, and the

		bufsize -= length;

in the loop was going negative - except bufsize is a size_t, so it
was wrapping to a very large number.

Then when we did
		ra_want = howmany(bufsize + mp->m_dirblksize,
				  mp->m_sb.sb_blocksize) - 1;

with that very large number, the (int) ra_want was coming out
negative, and a subsequent compare:

		if (1 + ra_want > map_blocks ...

was coming out -true- (negative int compare w/ uint) and we went
back to xfs_bmapi() for more, even though we did not need more,
and asked for 0 maps, and hit the ASSERT.

We have kind of a type mess here, but just keeping bufsize from
going negative is probably sufficient to avoid the problem.
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Alex Elder <aelder@sgi.com>

fs/xfs/xfs_dir2_leaf.c

@@ -854,6 +854,7 @@ xfs_dir2_leaf_getdents(
 			 */
 			ra_want = howmany(bufsize + mp->m_dirblksize,
 					  mp->m_sb.sb_blocksize) - 1;
+			ASSERT(ra_want >= 0);
 
 			/*
 			 * If we don't have as many as we want, and we haven't
@@ -1088,7 +1089,8 @@ xfs_dir2_leaf_getdents(
 		 */
 		ptr += length;
 		curoff += length;
-		bufsize -= length;
+		/* bufsize may have just been a guess; don't go negative */
+		bufsize = bufsize > length ? bufsize - length : 0;
 	}
 
 	/*