xfs: fix PAGE_MASK usage in xfs_free_file_space

In commit e53c4b59, I *tried* to teach xfs to force writeback when we
fzero/fpunch right up to EOF so that if EOF is in the middle of a page,
the post-EOF part of the page gets zeroed before we return to userspace.
Unfortunately, I missed the part where PAGE_MASK is ~(PAGE_SIZE - 1),
which means that we totally fail to zero if we're fpunching and EOF is
within the first page.  Worse yet, the same PAGE_MASK thinko plagues the
filemap_write_and_wait_range call, so we'd initiate writeback of the
entire file, which (mostly) masked the thinko.

Drop the tricky PAGE_MASK and replace it with correct usage of PAGE_SIZE
and the proper rounding macros.

Fixes: e53c4b59 ("xfs: ensure post-EOF zeroing happens after zeroing part of a file")
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/xfs_bmap_util.c

@@ -1126,9 +1126,9 @@ xfs_free_file_space(
 	 * page could be mmap'd and iomap_zero_range doesn't do that for us.
 	 * Writeback of the eof page will do this, albeit clumsily.
 	 */
-	if (offset + len >= XFS_ISIZE(ip) && ((offset + len) & PAGE_MASK)) {
+	if (offset + len >= XFS_ISIZE(ip) && offset_in_page(offset + len) > 0) {
 		error = filemap_write_and_wait_range(VFS_I(ip)->i_mapping,
-				(offset + len) & ~PAGE_MASK, LLONG_MAX);
+				round_down(offset + len, PAGE_SIZE), LLONG_MAX);
 	}
 
 	return error;