btrfs-progs: restore, fix page alignment issue for lzo compression

btrfs-progs: restore, fix page alignment issue for lzo compression

When runing restore under lzo compression, "bad compress length"
problems are encountered.
It is because there is a page alignment problem with the @decompress_lzo,
as follows:
		|------| |----|-| |------|...|------|
		  page         ^    page       page
			       |
			  3 bytes left

When lzo pages are compressed in memory, we will ensure that the 4 bytes
length header will not cross a page boundary.  There is a situation that
3 (or less) bytes are left at the end of a page, and then the 4 bytes
len is stored at the start of the next page.  But the @decompress_lzo
doesn't go to the start of the next page and continue to read the next 4
bytes which crosses two pages, so a random value is fetched as a "bad
compress length".

So we check page alignment every time before we are going to fetch the
next @len and after the former piece of data is decompressed.  If the
current page that we reach has less than 4 bytes left, then we should
fetch the next @len at the start of next page.

Signed-off-by: Gui Hecheng <guihc.fnst@cn.fujitsu.com>
[simplifed and moved into decompress_lzo]
Signed-off-by: David Sterba <dsterba@suse.cz>