GFS2: Fix direct IO write rounding error

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>