xz: use 128 MiB dictionary and force single-threaded mode

This only affects kernel image compression, not any other xz usage.

Desktop kernels on x86-64 are already around 60 MiB.  Using a dictionary
larger than 32 MiB should have no downsides nowadays as anyone building
the kernel should have plenty of RAM.  128 MiB dictionary needs 1346 MiB
of RAM with xz versions 5.0.x - 5.6.x in single-threaded mode.  On archs
that use xz_wrap.sh, kernel decompression is done in single-call mode so a
larger dictionary doesn't affect boot-time memory requirements.

xz >= 5.6.0 uses multithreaded mode by default which compresses slightly
worse than single-threaded mode.  Kernel compression rarely used more than
one thread anyway because with 32 MiB dictionary size the default block
size was 96 MiB in multithreaded mode.  So only a single thread was used
anyway unless the kernel was over 96 MiB.

Comparison to CONFIG_KERNEL_LZMA: It uses "lzma -9" which mapped to 32 MiB
dictionary in LZMA Utils 4.32.7 (the final release in 2008).  Nowadays the
lzma tool on most systems is from XZ Utils where -9 maps to 64 MiB
dictionary.  So using a 32 MiB dictionary with CONFIG_KERNEL_XZ may have
compressed big kernels slightly worse than the old LZMA option.

Comparison to CONFIG_KERNEL_ZSTD: zstd uses 128 MiB dictionary.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Lasse Collin <[email protected]>
Reviewed-by: Sam James <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Catalin Marinas <[email protected]>
Cc: Emil Renner Berthing <[email protected]>
Cc: Greg Kroah-Hartman <[email protected]>
Cc: Herbert Xu <[email protected]>
Cc: Joel Stanley <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Jubin Zhong <[email protected]>
Cc: Jules Maselbas <[email protected]>
Cc: Krzysztof Kozlowski <[email protected]>
Cc: Michael Ellerman <[email protected]>
Cc: Palmer Dabbelt <[email protected]>
Cc: Paul Walmsley <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Rui Li <[email protected]>
Cc: Simon Glass <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Will Deacon <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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