Fix ppp_deflate issues with recent zlib_inflate changes

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 <[email protected]>
Cc: Stefan Wenk <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>