fs: make block fiemap mapping length at least blocksize long

Some filesystems don't deal well with being asked to map less than
blocksize blocks (GFS2 for example).  Since we are always mapping at least
blocksize sections anyway, just make sure len is at least as big as a
blocksize so we don't trip up any filesystems.  Thanks,
Signed-off-by: Josef Bacik <josef@redhat.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ioctl.c

@@ -273,6 +273,13 @@ int __generic_block_fiemap(struct inode *inode,
 		len = isize;
 	}
 
+	/*
+	 * Some filesystems can't deal with being asked to map less than
+	 * blocksize, so make sure our len is at least block length.
+	 */
+	if (logical_to_blk(inode, len) == 0)
+		len = blk_to_logical(inode, 1);
+
 	start_blk = logical_to_blk(inode, start);
 	last_blk = logical_to_blk(inode, start + len - 1);