erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap()

In the Linux kernel, the following vulnerability has been resolved:

erofs: fix unsigned underflow in z_erofs_lz4_handle_overlap()

Some crafted images can have illegal (!partial_decoding &&

m_llen < m_plen) extents, and the LZ4 inplace decompression path

can be wrongly hit, but it cannot handle (outpages < inpages)

properly: "outpages - inpages" wraps to a large value and

the subsequent rq->out[] access reads past the decompressed_pages

array.

However, such crafted cases can correctly result in a corruption

report in the normal LZ4 non-inplace path.

Let's add an additional check to fix this for backporting.

Reproducible image (base64-encoded gzipped blob):

H4sIAJGR12kCA+3SPUoDQRgG4MkmkkZk8QRbRFIIi9hbpEjrHQI5ghfwCN5BLCzTGtLbBI+g

dilSJo1CnIm7GEXFxhT6PDDwfrs73/ywIQD/1ePD4r7Ou6ETsrq4mu7XcWfj++Pb58nJU/9i

PNtbjhan04/9GtX4qVYc814WDqt6FaX5s+ZwXXeq52lndT6IuVvlblytLMvh4Gzwaf90nsvz

2DF/21+20T/ldgp5s1jXRaN4t/8izsy/OUB6e/Qa79r+JwAAAAAAAL52vQVuGQAAAP6+my1w

ywAAAAAAAADwu14ATsEYtgBQAAA=

$ mount -t erofs -o cache_strategy=disabled foo.erofs /mnt

$ dd if=/mnt/data of=/dev/null bs=4096 count=1