erofs: fix EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS on some UP platforms

erofs: fix EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS on some UP platforms

CONFIG_NR_CPUS doesn't define on some UP platforms (e.g.  arm), so this
can cause make oldconfig to loop indefinitely when CONFIG_SMP=n:

  $ make ARCH=arm allmodconfig
  $ sed -i "/CONFIG_SMP=y/d" .config
  $ sed -i "/CONFIG_EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS.*/d" .config

  EROFS LZMA default maximum decompression streams (EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS) [0] (NEW)
  EROFS LZMA default maximum decompression streams (EROFS_FS_ZIP_LZMA_DEFAULT_MAX_STREAMS) [0] (NEW)
  ...

Let's guard NR_CPUS with SMP instead of using a hardcoded arbitrary CPU
uplimit here, similar to commit a3344078101c ("mm: make SPLIT_PTE_PTLOCKS
depend on SMP").

The initial report from SJ Park was for m68k [1] (m68k is the only arch
without NR_CPUS in Kconfig), and that got fixed in commit 1fd495ef09ee
("m68k: Define NR_CPUS to 1")

Reported-by: SJ Park <sj@kernel.org>
Link: https://lore.kernel.org/all/anuyFHLUGDjZWY4K@XiangdeMacBook-Pro.local/T/#u
Closes: https://lore.kernel.org/r/20260728065447.91511-1-sj@kernel.org
Reported-by: Guenter Roeck <groeck7@gmail.com>
Closes: https://lore.kernel.org/r/87853c96-cc8f-49e6-81b1-02bfe409e372@roeck-us.net
Fixes: c9b47e6b2311 ("erofs: cap LZMA stream pool size")
Signed-off-by: Gao Xiang <xiang@kernel.org>
Tested-by: SJ Park <sj@kernel.org>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>