xfs: ensure f_ffree returned by statfs() is non-negative

Because of delayed updates to sb_icount field in the super block, it
is possible to allocate over maxicount number of inodes.  This
causes the arithmetic to calculate a negative number of free inodes
in user commands like df or stat -f.

Since maxicount is a somewhat arbitrary number, a slight over
allocation is not critical but user commands should be displayed as
0 or greater and never go negative.  To do this the value in the
stats buffer f_ffree is capped to never go negative.

[ Modified to use max_t as per Christoph's comment. ]
Signed-off-by: NStu Brodsky <sbrodsky@sgi.com>
Signed-off-by: NDave Chinner <dchinner@redhat.com>

fs/xfs/linux-2.6/xfs_super.c

@@ -1226,6 +1226,7 @@ xfs_fs_statfs(
 	struct xfs_inode	*ip = XFS_I(dentry->d_inode);
 	__uint64_t		fakeinos, id;
 	xfs_extlen_t		lsize;
+	__int64_t		ffree;
 
 	statp->f_type = XFS_SB_MAGIC;
 	statp->f_namelen = MAXNAMELEN - 1;
@@ -1249,7 +1250,11 @@ xfs_fs_statfs(
 		statp->f_files = min_t(typeof(statp->f_files),
 					statp->f_files,
 					mp->m_maxicount);
-	statp->f_ffree = statp->f_files - (sbp->sb_icount - sbp->sb_ifree);
+
+	/* make sure statp->f_ffree does not underflow */
+	ffree = statp->f_files - (sbp->sb_icount - sbp->sb_ifree);
+	statp->f_ffree = max_t(__int64_t, ffree, 0);
+
 	spin_unlock(&mp->m_sb_lock);
 
 	if ((ip->i_d.di_flags & XFS_DIFLAG_PROJINHERIT) ||