ocfs2: use s_maxbytes directly in ocfs2_change_file_space()

There's no need to recalculate things via ocfs2_max_file_offset() as we've
already done that to fill s_maxbytes, so use that instead. We can also
un-export ocfs2_max_file_offset() then.

Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>