GFS2: Fix direct IO write rounding error

The fsx test in xfstests was failing because it was using direct IO
writes which were using a bad calculation. It was using
loff_t lstart = offset & (PAGE_CACHE_SIZE - 1); when it should be
loff_t lstart = offset & ~(PAGE_CACHE_SIZE - 1);
Thus, the write at offset 0x67e00 was calculating lstart to be
0xe00, the address of our corruption. Instead, it should have been
0x67000. This patch fixes the calculation.
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Acked-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/aops.c

@@ -1082,7 +1082,7 @@ static ssize_t gfs2_direct_IO(struct kiocb *iocb, struct iov_iter *iter,
 	 * the first place, mapping->nr_pages will always be zero.
 	 */
 	if (mapping->nrpages) {
-		loff_t lstart = offset & (PAGE_CACHE_SIZE - 1);
+		loff_t lstart = offset & ~(PAGE_CACHE_SIZE - 1);
 		loff_t len = iov_iter_count(iter);
 		loff_t end = PAGE_ALIGN(offset + len) - 1;