ext4: properly align shifted xattrs when expanding inodes

We did not count with the padding of xattr value when computing desired
shift of xattrs in the inode when expanding i_extra_isize. As a result
we could create unaligned start of inline xattrs. Account for alignment
properly.

CC: stable@vger.kernel.org  # 4.4.x-
Signed-off-by: Jan Kara <jack@suse.cz>