crypto: deflate - fix spurious -ENOSPC

The code in deflate_decompress_one may erroneously return -ENOSPC even if
it didn't run out of output space. The error happens under this
condition:

- Suppose that there are two input pages, the compressed data fits into
the first page and the zlib checksum is placed in the second page.

- The code iterates over the first page, decompresses the data and fully
fills the destination buffer, zlib_inflate returns Z_OK becuse zlib
hasn't seen the checksum yet.

- The outer do-while loop is iterated again, acomp_walk_next_src sets the
input parameters to the second page containing the checksum.

- We go into the inner do-while loop, execute "dcur =
acomp_walk_next_dst(&walk);". "dcur" is zero, so we break out of the
loop and return -ENOSPC, despite the fact that the decompressed data
fit into the destination buffer.

In order to fix this bug, this commit changes the logic when to report
the -ENOSPC error. We report the error if the destination buffer is empty
*and* if zlib_inflate didn't make any progress consuming the input
buffer. If zlib_inflate consumes the trailing checksum, we see that it
made progress and we will not return -ENOSPC.

Fixes: 08cabc7d3c86 ("crypto: deflate - Convert to acomp")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>