[PATCH] Add a gzip fastpath for the xmalloc readers, v3

[Lauri Kasanen]

v3: Use the aux struct as requested by Denys
v2: Add missing check on open

The performance and number of processes for a "depmod -a" with gzipped
modules was abysmal. This patch adds a fast path without fork for well-
behaved gzip files, benefiting all users of xmalloc_open_zipped_read_close.

"modinfo radeon.ko.gz", a single-file reader, got 30% faster.
"depmod -a", which used to fork over 800 times, got 20% faster. And of course
a whole lot less processes -> much saved RAM.

function                                             old     new   delta
inflate_unzip_internal                              2304    2521    +217
xmalloc_open_zipped_read_close                        73     201    +128
unpack_gz_stream                                     567     570      +3
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 3/0 up/down: 348/0)             Total: 348 bytes

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