ext4: keep existing extra fields when inode expands

ext4_expand_extra_isize() should clear only space between old and new
size.

Fixes: 6dd4ee7c

 # v2.6.23
Cc: stable@vger.kernel.org
Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>