gfs2: Support negative atimes

When inodes are read from disk, GFS2 will only update in-memory atimes
older than the on-disk atimes; this prevents atimes from going
backwards.  The atimes of newly allocated inodes are initialized to 0.
This means that when an atime is explicitly set to a negative value,
this value will not persist.

Fix by setting the atime of newly allocated inodes to the lowest
possible value instead of 0.

Fixes xfstest generic/258.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>

fs/gfs2/inode.c

@@ -189,7 +189,8 @@ struct inode *gfs2_inode_lookup(struct super_block *sb, unsigned int type,
 
 		gfs2_set_iop(inode);
 
-		inode->i_atime.tv_sec = 0;
+		/* Lowest possible timestamp; will be overwritten in gfs2_dinode_in. */
+		inode->i_atime.tv_sec = 1LL << (8 * sizeof(inode->i_atime.tv_sec) - 1);
 		inode->i_atime.tv_nsec = 0;
 
 		unlock_new_inode(inode);