GFS2: Fix typo in gfs_page_mkwrite()

There is a typo in gfs2_page_mkwrite()

gfs2_write_alloc_required() expects pos to be the offset in bytes. However,
instead of the page index being shifted by by PAGE_CACHE_SHIFT, it was shifted
by (PAGE_CACHE_SIZE - inode->i_blkbits).  This patch simply shifts the page
index by the proper amount.
Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/ops_file.c

@@ -342,7 +342,7 @@ static int gfs2_page_mkwrite(struct vm_area_struct *vma, struct page *page)
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_sbd *sdp = GFS2_SB(inode);
 	unsigned long last_index;
-	u64 pos = page->index << (PAGE_CACHE_SIZE - inode->i_blkbits);
+	u64 pos = page->index << PAGE_CACHE_SHIFT;
 	unsigned int data_blocks, ind_blocks, rblocks;
 	int alloc_required = 0;
 	struct gfs2_holder gh;