[PATCH] Improve performance for parallel compression with zlib.

compress2() allocates a buffer, compresses a input data and
deallocates the buffer in each call. makedumpfile has to call
compress2() for each page, it can cause big performance
degradation due to many page faults. This issue will be
especially apparent in the case of multi thread compression
since per-thread arena is easy to be grown and trimmed compared
with main arena.

Fortunally, the zlib functions called in compress2() are global,
it's easy to extract the allocation and deallocation part from
compress2().

Signed-off-by: Atsushi Kumagai <ats-kumagai@wm.jp.nec.com>