Fix ppp_deflate issues with recent zlib_inflate changes

The last zlib_inflate update broke certain corner cases for ppp_deflate
decompression handling.  This patch fixes some logic to make things work
properly again.  Users other than ppp_deflate (the only Z_PACKET_FLUSH
user) should be unaffected.

Fixes bug 8405 (confirmed by Stefan)

Signed-off-by: Richard Purdie <rpurdie@rpsys.net>
Cc: Stefan Wenk <stefan.wenk@gmx.at>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>